diff --git a/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
index 6c9a8da61f..caa4065a6b 100644
--- a/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -40,6 +40,7 @@
 use TYPO3\PharStreamWrapper\Behavior as PharStreamWrapperBehavior;
 use TYPO3\PharStreamWrapper\PharStreamWrapper;
 
+// @todo: remove this class alias once Drupal is running Symfony 5.3 or higher.
 class_alias(KernelEvent::class, 'Symfony\Component\HttpKernel\Event\KernelEvent', TRUE);
 
 /**
diff --git a/core/lib/Drupal/Core/Http/KernelEvent.php b/core/lib/Drupal/Core/Http/KernelEvent.php
index 42ea9672cf..cdefd65c60 100644
--- a/core/lib/Drupal/Core/Http/KernelEvent.php
+++ b/core/lib/Drupal/Core/Http/KernelEvent.php
@@ -3,9 +3,6 @@
 // @codingStandardsIgnoreFile
 // cspell:disable
 
-/*
- * Symfony 6 bridge.
- */
 
 namespace Drupal\Core\Http;
 
@@ -14,9 +11,13 @@
 use Symfony\Component\HttpKernel\HttpKernelInterface;
 
 /**
- * Base class for events thrown in the HttpKernel component.
+/*
+ * Symfony 6 bridge.
  *
- * @author Bernhard Schussek <bschussek@gmail.com>
+ * This is a copy of Symfony\Component\HttpKernel\Event\KernelEvent
+ * with two changes: adding an ::isMainRequest() method for forward
+ * compatibility with Symfony 5.3+, and issuing a deprecation message from
+ * ::isMasterRequest().
  */
 class KernelEvent extends Event
 {
diff --git a/core/tests/Drupal/KernelTests/Core/DrupalKernel/DrupalKernelTest.php b/core/tests/Drupal/KernelTests/Core/DrupalKernel/DrupalKernelTest.php
index c610b7a972..659a8cef87 100644
--- a/core/tests/Drupal/KernelTests/Core/DrupalKernel/DrupalKernelTest.php
+++ b/core/tests/Drupal/KernelTests/Core/DrupalKernel/DrupalKernelTest.php
@@ -61,6 +61,8 @@ protected function getTestKernel(Request $request, array $modules_enabled = NULL
 
   /**
    * Tests KernelEvent class_alias() override.
+   *
+   * @todo: remove this test once Drupal is using Symfony 5.3 or higher.
    */
   public function testKernelEvent() {
     $request = Request::createFromGlobals();