diff --git a/core/modules/page_cache/tests/src/Functional/PageCacheTest.php b/core/modules/page_cache/tests/src/Functional/PageCacheTest.php
index e14cbdd42bbb38b2ba3825088ea5a1b18c9c8496..f1c404d0ab90b521c923fc248be17954a968634c 100644
--- a/core/modules/page_cache/tests/src/Functional/PageCacheTest.php
+++ b/core/modules/page_cache/tests/src/Functional/PageCacheTest.php
@@ -247,7 +247,7 @@ public function testPageCache() {
     // Fill the cache.
     $this->drupalGet('system-test/set-header', ['query' => ['name' => 'Foo', 'value' => 'bar']]);
     $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'MISS', 'Page was not cached.');
-    $this->assertContains('cookie', explode(',', strtolower($this->drupalGetHeader('Vary'))), 'Vary header was sent.', TRUE);
+    $this->assertContains('cookie', explode(',', strtolower($this->drupalGetHeader('Vary'))), 'Vary header was sent.');
     // Symfony's Response logic determines a specific order for the subvalues
     // of the Cache-Control header, even if they are explicitly passed in to
     // the response header bag in a different order.
@@ -258,7 +258,7 @@ public function testPageCache() {
     // Check cache.
     $this->drupalGet('system-test/set-header', ['query' => ['name' => 'Foo', 'value' => 'bar']]);
     $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'HIT', 'Page was cached.');
-    $this->assertContains('cookie', explode(',', strtolower($this->drupalGetHeader('Vary'))), 'Vary header was sent.', TRUE);
+    $this->assertContains('cookie', explode(',', strtolower($this->drupalGetHeader('Vary'))), 'Vary header was sent.');
     $this->assertEqual($this->drupalGetHeader('Cache-Control'), 'max-age=300, public', 'Cache-Control header was sent.');
     $this->assertEqual($this->drupalGetHeader('Expires'), 'Sun, 19 Nov 1978 05:00:00 GMT', 'Expires header was sent.');
     $this->assertEqual($this->drupalGetHeader('Foo'), 'bar', 'Custom header was sent.');
@@ -274,7 +274,7 @@ public function testPageCache() {
     $this->drupalLogin($user);
     $this->drupalGet('system-test/set-header', ['query' => ['name' => 'Foo', 'value' => 'bar']]);
     $this->assertNull($this->drupalGetHeader('X-Drupal-Cache'), 'Caching was bypassed.');
-    $this->assertStringNotContainsString('cookie', $this->drupalGetHeader('Vary'), 'Vary: Cookie header was not sent.', TRUE);
+    $this->assertStringNotContainsStringIgnoringCase('cookie', $this->drupalGetHeader('Vary'), 'Vary: Cookie header was not sent.');
     $this->assertEqual($this->drupalGetHeader('Cache-Control'), 'must-revalidate, no-cache, private', 'Cache-Control header was sent.');
     $this->assertEqual($this->drupalGetHeader('Expires'), 'Sun, 19 Nov 1978 05:00:00 GMT', 'Expires header was sent.');
     $this->assertEqual($this->drupalGetHeader('Foo'), 'bar', 'Custom header was sent.');
@@ -441,13 +441,13 @@ public function testPageCacheWithoutVaryCookie() {
     // Fill the cache.
     $this->drupalGet('');
     $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'MISS', 'Page was not cached.');
-    $this->assertStringNotContainsString('cookie', $this->drupalGetHeader('Vary'), 'Vary: Cookie header was not sent.', TRUE);
+    $this->assertStringNotContainsStringIgnoringCase('cookie', $this->drupalGetHeader('Vary'), 'Vary: Cookie header was not sent.');
     $this->assertEqual($this->drupalGetHeader('Cache-Control'), 'max-age=300, public', 'Cache-Control header was sent.');
 
     // Check cache.
     $this->drupalGet('');
     $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'HIT', 'Page was cached.');
-    $this->assertStringNotContainsString('cookie', $this->drupalGetHeader('Vary'), 'Vary: Cookie header was not sent.', TRUE);
+    $this->assertStringNotContainsStringIgnoringCase('cookie', $this->drupalGetHeader('Vary'), 'Vary: Cookie header was not sent.');
     $this->assertEqual($this->drupalGetHeader('Cache-Control'), 'max-age=300, public', 'Cache-Control header was sent.');
   }
 
diff --git a/core/tests/Drupal/Tests/Traits/PHPUnit8Warnings.php b/core/tests/Drupal/Tests/Traits/PHPUnit8Warnings.php
index ba91b651b60b6d78538d450f9b7e4f075d27f613..e011499fabc96def4c763c1102dcadc72490249d 100644
--- a/core/tests/Drupal/Tests/Traits/PHPUnit8Warnings.php
+++ b/core/tests/Drupal/Tests/Traits/PHPUnit8Warnings.php
@@ -28,8 +28,6 @@ trait PHPUnit8Warnings {
     'getObjectAttribute() is deprecated and will be removed in PHPUnit 9.',
     'assertAttributeEquals() is deprecated and will be removed in PHPUnit 9.',
     'assertAttributeInstanceOf() is deprecated and will be removed in PHPUnit 9.',
-    'The optional $ignoreCase parameter of assertContains() is deprecated and will be removed in PHPUnit 9.',
-    'The optional $ignoreCase parameter of assertNotContains() is deprecated and will be removed in PHPUnit 9.',
   ];
 
   /**