diff --git a/core/lib/Drupal/Core/Entity/ContentEntityType.php b/core/lib/Drupal/Core/Entity/ContentEntityType.php
index 5be34c9ba553a97c6940a1fe71151d141c8190f2..38714051762d844f3bac24406e2a5d9ddd91f2bb 100644
--- a/core/lib/Drupal/Core/Entity/ContentEntityType.php
+++ b/core/lib/Drupal/Core/Entity/ContentEntityType.php
@@ -14,6 +14,18 @@ class ContentEntityType extends EntityType implements ContentEntityTypeInterface
    */
   protected $revision_metadata_keys = [];
 
+  /**
+   * The required revision metadata keys.
+   *
+   * This property should only be filled in the constructor. This ensures that
+   * only new instances get newly added required revision metadata keys.
+   * Unserialized objects will only retrieve the keys that they already have
+   * been cached with.
+   *
+   * @var array
+   */
+  protected $requiredRevisionMetadataKeys = [];
+
   /**
    * {@inheritdoc}
    */
@@ -25,9 +37,23 @@ public function __construct($definition) {
       'view_builder' => 'Drupal\Core\Entity\EntityViewBuilder',
     ];
 
-    $this->revision_metadata_keys += [
-      'revision_default' => 'revision_default',
-    ];
+    // Only new instances should provide the required revision metadata keys.
+    // The cached instances should return only what already has been stored
+    // under the property $revision_metadata_keys. The BC layer in
+    // ::getRevisionMetadataKeys() has to detect if the revision metadata keys
+    // have been provided by the entity type annotation, therefore we add keys
+    // to the property $requiredRevisionMetadataKeys only if those keys aren't
+    // set in the entity type annotation.
+    if (!isset($this->revision_metadata_keys['revision_default'])) {
+      $this->requiredRevisionMetadataKeys['revision_default'] = 'revision_default';
+    }
+
+    // Add the required revision metadata fields here instead in the getter
+    // method, so that they are serialized as part of the object even if the
+    // getter method doesn't get called. This allows the list to be further
+    // extended. Only new instances of the class will contain the new list,
+    // while the cached instances contain the previous version of the list.
+    $this->revision_metadata_keys += $this->requiredRevisionMetadataKeys;
   }
 
   /**
@@ -59,7 +85,7 @@ protected function checkStorageClass($class) {
   public function getRevisionMetadataKeys($include_backwards_compatibility_field_names = TRUE) {
     // Provide backwards compatibility in case the revision metadata keys are
     // not defined in the entity annotation.
-    if (!$this->revision_metadata_keys && $include_backwards_compatibility_field_names) {
+    if ((!$this->revision_metadata_keys || ($this->revision_metadata_keys == $this->requiredRevisionMetadataKeys)) && $include_backwards_compatibility_field_names) {
       $base_fields = \Drupal::service('entity_field.manager')->getBaseFieldDefinitions($this->id());
       if ((isset($base_fields['revision_uid']) && $revision_user = 'revision_uid') || (isset($base_fields['revision_user']) && $revision_user = 'revision_user')) {
         @trigger_error('The revision_user revision metadata key is not set.', E_USER_DEPRECATED);
diff --git a/core/lib/Drupal/Core/Entity/EntityFieldManager.php b/core/lib/Drupal/Core/Entity/EntityFieldManager.php
index c19956491d128b34f7c19db9c4613719910de227..025d92cca1ebaafe12676fb80c16f45dfc6a472b 100644
--- a/core/lib/Drupal/Core/Entity/EntityFieldManager.php
+++ b/core/lib/Drupal/Core/Entity/EntityFieldManager.php
@@ -224,7 +224,9 @@ protected function buildBaseFieldDefinitions($entity_type_id) {
 
     // Make sure that revisionable entity types are correctly defined.
     if ($entity_type->isRevisionable()) {
-      $field_name = $entity_type->getRevisionMetadataKey('revision_default');
+      // Disable the BC layer to prevent a recursion, this only needs the
+      // revision_default key that is always set.
+      $field_name = $entity_type->getRevisionMetadataKeys(FALSE)['revision_default'];
       $base_field_definitions[$field_name] = BaseFieldDefinition::create('boolean')
         ->setLabel($this->t('Default revision'))
         ->setDescription($this->t('A flag indicating whether this was a default revision when it was saved.'))
diff --git a/core/modules/system/system.install b/core/modules/system/system.install
index 143fed847d0fc70d5baac620137fcd98c0473b79..18383264519cd28e728c0b450f65c16e40cbe2ce 100644
--- a/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -2092,9 +2092,23 @@ function system_update_8501() {
       // also to code using the latest installed definition.
       $installed_entity_type = $definition_update_manager->getEntityType($entity_type_id);
       $revision_metadata_keys = $installed_entity_type->get('revision_metadata_keys');
-      $revision_metadata_keys['revision_default'] = $field_name;
-      $installed_entity_type->set('revision_metadata_keys', $revision_metadata_keys);
-      $definition_update_manager->updateEntityType($installed_entity_type);
+
+      if (!isset($revision_metadata_keys['revision_default'])) {
+        // Update the property holding the required revision metadata keys,
+        // which is used by the BC layer for retrieving the revision metadata
+        // keys.
+        // @see \Drupal\Core\Entity\ContentEntityType::getRevisionMetadataKeys().
+        $required_revision_metadata_keys = $installed_entity_type->get('requiredRevisionMetadataKeys');
+        $required_revision_metadata_keys['revision_default'] = $field_name;
+        $installed_entity_type->set('requiredRevisionMetadataKeys', $required_revision_metadata_keys);
+
+        // Update the revision metadata keys to add the new required revision
+        // metadata key "revision_default".
+        $revision_metadata_keys['revision_default'] = $required_revision_metadata_keys['revision_default'];
+        $installed_entity_type->set('revision_metadata_keys', $revision_metadata_keys);
+
+        $definition_update_manager->updateEntityType($installed_entity_type);
+      }
 
       $storage_definition = BaseFieldDefinition::create('boolean')
         ->setLabel(t('Default revision'))
@@ -2110,5 +2124,14 @@ function system_update_8501() {
       $definition_update_manager
         ->installFieldStorageDefinition($field_name, $entity_type_id, $entity_type_id, $storage_definition);
     }
+    else {
+      $variables = ['@entity_type_label' => $entity_type->getLabel()];
+      if ($field_name === 'revision_default') {
+        \Drupal::logger('system')->error('An existing "Default revision" field was found for the @entity_type_label entity type, but no "revision_default" revision metadata key was found in its definition.', $variables);
+      }
+      else {
+        \Drupal::logger('system')->error('An existing "Default revision" field was found for the @entity_type_label entity type.', $variables);
+      }
+    }
   }
 }
diff --git a/core/modules/system/tests/modules/entity_test_revlog/src/Entity/EntityTestMulWithRevisionLog.php b/core/modules/system/tests/modules/entity_test_revlog/src/Entity/EntityTestMulWithRevisionLog.php
index 5a17b858dc1f0bfc444d52b87617acf5a49e8181..cb9c7bbc0c7f58a90a327f340f59231632349df0 100644
--- a/core/modules/system/tests/modules/entity_test_revlog/src/Entity/EntityTestMulWithRevisionLog.php
+++ b/core/modules/system/tests/modules/entity_test_revlog/src/Entity/EntityTestMulWithRevisionLog.php
@@ -5,6 +5,9 @@
 /**
  * Defines the test entity class.
  *
+ * This entity type does not define revision_metadata_keys on purpose to test
+ * the BC layer.
+ *
  * @ContentEntityType(
  *   id = "entity_test_mul_revlog",
  *   label = @Translation("Test entity - data table, revisions log"),
@@ -21,11 +24,6 @@
  *     "label" = "name",
  *     "langcode" = "langcode",
  *   },
- *   revision_metadata_keys = {
- *     "revision_user" = "revision_user",
- *     "revision_created" = "revision_created",
- *     "revision_log_message" = "revision_log_message"
- *   },
  * )
  */
 class EntityTestMulWithRevisionLog extends EntityTestWithRevisionLog {
diff --git a/core/modules/system/tests/src/Functional/Entity/Update/MoveRevisionMetadataFieldsUpdateTest.php b/core/modules/system/tests/src/Functional/Entity/Update/MoveRevisionMetadataFieldsUpdateTest.php
index 8634b16d6e35a0426028238523bd1bc7f668cf4c..b4888db0985ade5c44f3c6353c3a5da4a0ebd65f 100644
--- a/core/modules/system/tests/src/Functional/Entity/Update/MoveRevisionMetadataFieldsUpdateTest.php
+++ b/core/modules/system/tests/src/Functional/Entity/Update/MoveRevisionMetadataFieldsUpdateTest.php
@@ -2,6 +2,7 @@
 
 namespace Drupal\Tests\system\Functional\Entity\Update;
 
+use Drupal\Core\Entity\ContentEntityType;
 use Drupal\FunctionalTests\Update\UpdatePathTestBase;
 use Drupal\views\Entity\View;
 
@@ -79,4 +80,156 @@ public function testSystemUpdate8400() {
     }
   }
 
+  /**
+   * Tests the addition of required revision metadata keys.
+   *
+   * This test ensures that already cached entity instances will only return the
+   * required revision metadata keys they have been cached with and only new
+   * instances will return all the new required revision metadata keys.
+   */
+  public function testAddingRequiredRevisionMetadataKeys() {
+    // Ensure that cached entity types without required revision metadata keys
+    // will not return any of the newly added required revision metadata keys.
+    // Contains no revision metadata keys and the property holding the required
+    // metadata keys is empty, the entity type id is "entity_test_mul_revlog".
+    $cached_with_no_metadata_keys = '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';
+    /** @var \Drupal\Tests\system\Functional\Entity\Update\TestRevisionMetadataBcLayerEntityType $entity_type */
+    $entity_type = unserialize(base64_decode($cached_with_no_metadata_keys));
+    $required_revision_metadata_keys_no_bc = [];
+    $this->assertEquals($required_revision_metadata_keys_no_bc, $entity_type->getRevisionMetadataKeys(FALSE));
+    $required_revision_metadata_keys_with_bc = $required_revision_metadata_keys_no_bc + [
+      'revision_user' => 'revision_user',
+      'revision_created' => 'revision_created',
+      'revision_log_message' => 'revision_log_message',
+    ];
+    $this->assertEquals($required_revision_metadata_keys_with_bc, $entity_type->getRevisionMetadataKeys(TRUE));
+
+    // Ensure that cached entity types with only one required revision metadata
+    // key will return only that one after a second required revision metadata
+    // key has been added.
+    // Contains one revision metadata key - revision_default which is also
+    // contained in the property holding the required revision metadata keys,
+    // the entity type id is "entity_test_mul_revlog".
+    $cached_with_metadata_key_revision_default = 'Tzo4MjoiRHJ1cGFsXFRlc3RzXHN5c3RlbVxGdW5jdGlvbmFsXEVudGl0eVxVcGRhdGVcVGVzdFJldmlzaW9uTWV0YWRhdGFCY0xheWVyRW50aXR5VHlwZSI6Mzk6e3M6MjU6IgAqAHJldmlzaW9uX21ldGFkYXRhX2tleXMiO2E6MTp7czoxNjoicmV2aXNpb25fZGVmYXVsdCI7czoxNjoicmV2aXNpb25fZGVmYXVsdCI7fXM6MzE6IgAqAHJlcXVpcmVkUmV2aXNpb25NZXRhZGF0YUtleXMiO2E6MTp7czoxNjoicmV2aXNpb25fZGVmYXVsdCI7czoxNjoicmV2aXNpb25fZGVmYXVsdCI7fXM6MTU6IgAqAHN0YXRpY19jYWNoZSI7YjoxO3M6MTU6IgAqAHJlbmRlcl9jYWNoZSI7YjoxO3M6MTk6IgAqAHBlcnNpc3RlbnRfY2FjaGUiO2I6MTtzOjE0OiIAKgBlbnRpdHlfa2V5cyI7YTo1OntzOjg6InJldmlzaW9uIjtzOjA6IiI7czo2OiJidW5kbGUiO3M6MDoiIjtzOjg6Imxhbmdjb2RlIjtzOjA6IiI7czoxNjoiZGVmYXVsdF9sYW5nY29kZSI7czoxNjoiZGVmYXVsdF9sYW5nY29kZSI7czoyOToicmV2aXNpb25fdHJhbnNsYXRpb25fYWZmZWN0ZWQiO3M6Mjk6InJldmlzaW9uX3RyYW5zbGF0aW9uX2FmZmVjdGVkIjt9czo1OiIAKgBpZCI7czoyMjoiZW50aXR5X3Rlc3RfbXVsX3JldmxvZyI7czoxNjoiACoAb3JpZ2luYWxDbGFzcyI7TjtzOjExOiIAKgBoYW5kbGVycyI7YTozOntzOjY6ImFjY2VzcyI7czo0NToiRHJ1cGFsXENvcmVcRW50aXR5XEVudGl0eUFjY2Vzc0NvbnRyb2xIYW5kbGVyIjtzOjc6InN0b3JhZ2UiO3M6NDY6IkRydXBhbFxDb3JlXEVudGl0eVxTcWxcU3FsQ29udGVudEVudGl0eVN0b3JhZ2UiO3M6MTI6InZpZXdfYnVpbGRlciI7czozNjoiRHJ1cGFsXENvcmVcRW50aXR5XEVudGl0eVZpZXdCdWlsZGVyIjt9czoxOToiACoAYWRtaW5fcGVybWlzc2lvbiI7TjtzOjI1OiIAKgBwZXJtaXNzaW9uX2dyYW51bGFyaXR5IjtzOjExOiJlbnRpdHlfdHlwZSI7czo4OiIAKgBsaW5rcyI7YTowOnt9czoxNzoiACoAbGFiZWxfY2FsbGJhY2siO047czoyMToiACoAYnVuZGxlX2VudGl0eV90eXBlIjtOO3M6MTI6IgAqAGJ1bmRsZV9vZiI7TjtzOjE1OiIAKgBidW5kbGVfbGFiZWwiO047czoxMzoiACoAYmFzZV90YWJsZSI7TjtzOjIyOiIAKgByZXZpc2lvbl9kYXRhX3RhYmxlIjtOO3M6MTc6IgAqAHJldmlzaW9uX3RhYmxlIjtOO3M6MTM6IgAqAGRhdGFfdGFibGUiO047czoxNToiACoAdHJhbnNsYXRhYmxlIjtiOjA7czoxOToiACoAc2hvd19yZXZpc2lvbl91aSI7YjowO3M6ODoiACoAbGFiZWwiO3M6MDoiIjtzOjE5OiIAKgBsYWJlbF9jb2xsZWN0aW9uIjtzOjA6IiI7czoxNzoiACoAbGFiZWxfc2luZ3VsYXIiO3M6MDoiIjtzOjE1OiIAKgBsYWJlbF9wbHVyYWwiO3M6MDoiIjtzOjE0OiIAKgBsYWJlbF9jb3VudCI7YTowOnt9czoxNToiACoAdXJpX2NhbGxiYWNrIjtOO3M6ODoiACoAZ3JvdXAiO047czoxNDoiACoAZ3JvdXBfbGFiZWwiO047czoyMjoiACoAZmllbGRfdWlfYmFzZV9yb3V0ZSI7TjtzOjI2OiIAKgBjb21tb25fcmVmZXJlbmNlX3RhcmdldCI7YjowO3M6MjI6IgAqAGxpc3RfY2FjaGVfY29udGV4dHMiO2E6MDp7fXM6MTg6IgAqAGxpc3RfY2FjaGVfdGFncyI7YToxOntpOjA7czo5OiJ0ZXN0X2xpc3QiO31zOjE0OiIAKgBjb25zdHJhaW50cyI7YTowOnt9czoxMzoiACoAYWRkaXRpb25hbCI7YTowOnt9czo4OiIAKgBjbGFzcyI7TjtzOjExOiIAKgBwcm92aWRlciI7TjtzOjIwOiIAKgBzdHJpbmdUcmFuc2xhdGlvbiI7Tjt9';
+    $entity_type = unserialize(base64_decode($cached_with_metadata_key_revision_default));
+    $required_revision_metadata_keys_no_bc = [
+      'revision_default' => 'revision_default',
+    ];
+    $this->assertEquals($required_revision_metadata_keys_no_bc, $entity_type->getRevisionMetadataKeys(FALSE));
+    $required_revision_metadata_keys_with_bc = $required_revision_metadata_keys_no_bc + [
+        'revision_user' => 'revision_user',
+        'revision_created' => 'revision_created',
+        'revision_log_message' => 'revision_log_message',
+      ];
+    $this->assertEquals($required_revision_metadata_keys_with_bc, $entity_type->getRevisionMetadataKeys(TRUE));
+
+    // Ensure that newly instantiated entity types will return the two required
+    // revision metadata keys.
+    $entity_type = new TestRevisionMetadataBcLayerEntityType(['id' => 'test']);
+    $required_revision_metadata_keys = [
+      'revision_default' => 'revision_default',
+      'second_required_key' => 'second_required_key',
+    ];
+    $this->assertEquals($required_revision_metadata_keys, $entity_type->getRevisionMetadataKeys(FALSE));
+
+    // Load an entity type from the cache with no revision metadata keys in the
+    // annotation.
+    $entity_last_installed_schema_repository = \Drupal::service('entity.last_installed_schema.repository');
+    $entity_type = $entity_last_installed_schema_repository->getLastInstalledDefinition('entity_test_mul_revlog');
+    $revision_metadata_keys = [];
+    $this->assertEquals($revision_metadata_keys, $entity_type->getRevisionMetadataKeys(FALSE));
+    $revision_metadata_keys = [
+      'revision_user' => 'revision_user',
+      'revision_created' => 'revision_created',
+      'revision_log_message' => 'revision_log_message'
+    ];
+    $this->assertEquals($revision_metadata_keys, $entity_type->getRevisionMetadataKeys(TRUE));
+
+    // Load an entity type without using the cache with no revision metadata
+    // keys in the annotation.
+    $entity_type_manager = \Drupal::entityTypeManager();
+    $entity_type_manager->useCaches(FALSE);
+    $entity_type = $entity_type_manager->getDefinition('entity_test_mul_revlog');
+    $revision_metadata_keys = [
+      'revision_default' => 'revision_default',
+    ];
+    $this->assertEquals($revision_metadata_keys, $entity_type->getRevisionMetadataKeys(FALSE));
+    $revision_metadata_keys = [
+      'revision_user' => 'revision_user',
+      'revision_created' => 'revision_created',
+      'revision_log_message' => 'revision_log_message',
+      'revision_default' => 'revision_default',
+    ];
+    $this->assertEquals($revision_metadata_keys, $entity_type->getRevisionMetadataKeys(TRUE));
+
+    // Ensure that the BC layer will not be triggered if one of the required
+    // revision metadata keys is defined in the annotation.
+    $definition = [
+      'id' => 'entity_test_mul_revlog',
+      'revision_metadata_keys' => [
+        'revision_default' => 'revision_default'
+      ],
+    ];
+    $entity_type = new ContentEntityType($definition);
+    $revision_metadata_keys = [
+      'revision_default' => 'revision_default',
+    ];
+    $this->assertEquals($revision_metadata_keys, $entity_type->getRevisionMetadataKeys(TRUE));
+
+    // Ensure that the BC layer will be triggered if no revision metadata keys
+    // have been defined in the annotation.
+    $definition = [
+      'id' => 'entity_test_mul_revlog',
+    ];
+    $entity_type = new ContentEntityType($definition);
+    $revision_metadata_keys = [
+      'revision_default' => 'revision_default',
+      'revision_user' => 'revision_user',
+      'revision_created' => 'revision_created',
+      'revision_log_message' => 'revision_log_message',
+    ];
+    $this->assertEquals($revision_metadata_keys, $entity_type->getRevisionMetadataKeys(TRUE));
+  }
+
+  /**
+   * Tests that the revision metadata key BC layer was updated correctly.
+   */
+  public function testSystemUpdate8501() {
+    $this->runUpdates();
+
+    /** @var \Drupal\Core\Entity\EntityDefinitionUpdateManagerInterface $definition_update_manager */
+    $definition_update_manager = $this->container->get('entity.definition_update_manager');
+    foreach (['block_content', 'node'] as $entity_type_id) {
+      $installed_entity_type = $definition_update_manager->getEntityType($entity_type_id);
+      $revision_metadata_keys = $installed_entity_type->get('revision_metadata_keys');
+      $this->assertTrue(isset($revision_metadata_keys['revision_default']));
+      $required_revision_metadata_keys = $installed_entity_type->get('requiredRevisionMetadataKeys');
+      $this->assertTrue(isset($required_revision_metadata_keys['revision_default']));
+    }
+  }
+
+}
+
+/**
+ * Test entity type class for adding new required revision metadata keys.
+ */
+class TestRevisionMetadataBcLayerEntityType extends ContentEntityType {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function __construct($definition) {
+    // Only new instances should provide the required revision metadata keys.
+    // The cached instances should return only what already has been stored
+    // under the property $revision_metadata_keys. The BC layer in
+    // ::getRevisionMetadataKeys() has to detect if the revision metadata keys
+    // have been provided by the entity type annotation, therefore we add keys
+    // to the property $requiredRevisionMetadataKeys only if those keys aren't
+    // set in the entity type annotation.
+    if (!isset($definition['revision_metadata_keys']['second_required_key'])) {
+      $this->requiredRevisionMetadataKeys['second_required_key'] = 'second_required_key';
+    }
+    parent::__construct($definition);
+  }
+
 }