diff --git a/core/lib/Drupal/Component/Render/FormattableMarkup.php b/core/lib/Drupal/Component/Render/FormattableMarkup.php
index d9cb205fe64e604ca4bd281c7ee9539dcccfac84..4d6cc59ffdc555acb2337cd33e7c7278e41d8d6b 100644
--- a/core/lib/Drupal/Component/Render/FormattableMarkup.php
+++ b/core/lib/Drupal/Component/Render/FormattableMarkup.php
@@ -233,13 +233,8 @@ protected static function placeholderFormat($string, array $args) {
           break;
 
         default:
-          // Deprecate support for random variables that won't be replaced.
-          if (ctype_alpha($key[0]) && strpos($string, $key) === FALSE) {
-            @trigger_error(sprintf('Support for keys without a placeholder prefix is deprecated in Drupal 9.1.0 and will be removed in Drupal 10.0.0. Invalid placeholder (%s) with string: "%s"', $key, $string), E_USER_DEPRECATED);
-          }
-          else {
-            trigger_error(sprintf('Invalid placeholder (%s) with string: "%s"', $key, $string), E_USER_WARNING);
-          }
+          // Warn for random variables that won't be replaced.
+          trigger_error(sprintf('Invalid placeholder (%s) with string: "%s"', $key, $string), E_USER_WARNING);
           // No replacement possible therefore we can discard the argument.
           unset($args[$key]);
           break;
diff --git a/core/tests/Drupal/Tests/Component/Render/FormattableMarkupTest.php b/core/tests/Drupal/Tests/Component/Render/FormattableMarkupTest.php
index fc285287e1e3546e5bcbc0f0b5b97500c77d369c..4c008a9b652e6c5c2d54087562e45b200ca461dc 100644
--- a/core/tests/Drupal/Tests/Component/Render/FormattableMarkupTest.php
+++ b/core/tests/Drupal/Tests/Component/Render/FormattableMarkupTest.php
@@ -99,17 +99,8 @@ public function providerTestUnexpectedPlaceholder() {
       // Ensure that where the placeholder is located in the string is
       // irrelevant.
       ['placeholder', ['placeholder' => 'replaced'], E_USER_WARNING, 'Invalid placeholder (placeholder) with string: "placeholder"'],
+      ['No replacements', ['foo' => 'bar'], E_USER_WARNING, 'Invalid placeholder (foo) with string: "No replacements"'],
     ];
   }
 
-  /**
-   * @group legacy
-   */
-  public function testNoReplacementUnsupportedVariable() {
-    $this->expectDeprecation('Support for keys without a placeholder prefix is deprecated in Drupal 9.1.0 and will be removed in Drupal 10.0.0. Invalid placeholder (foo) with string: "No replacements"');
-    $markup = new FormattableMarkup('No replacements', ['foo' => 'bar']);
-    // Cast it to a string which will generate the deprecation notice.
-    $output = (string) $markup;
-  }
-
 }