diff -u b/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
--- b/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -480,7 +480,15 @@
     FileCacheFactory::setConfiguration($configuration);
     FileCacheFactory::setPrefix(Settings::getApcuPrefix('file_cache', $this->root));
 
-    $this->bootstrapContainer = new $this->bootstrapContainerClass(Settings::get('bootstrap_container_definition', $this->defaultBootstrapContainerDefinition));
+    $bootstrap_container_definition = Settings::get('bootstrap_container_definition', $this->defaultBootstrapContainerDefinition);
+    // BC layer for ensuring that the PHP serialization service is present in
+    // case the bootstrap container definition is overridden and does not yet
+    // contain it.
+    if (!isset($bootstrap_container_definition['serialization.phpserialize'])) {
+      @trigger_error('The "serialization.phpserialize" service is required for the bootstrap container. See: https://www.drupal.org/node/3014688', E_USER_DEPRECATED);
+      $bootstrap_container_definition['serialization.phpserialize'] = ['class' => 'Drupal\Component\Serialization\PhpSerialize'];
+    }
+    $this->bootstrapContainer = new $this->bootstrapContainerClass($bootstrap_container_definition);
 
     // Initialize the container.
     $this->initializeContainer();
only in patch2:
unchanged:
--- a/core/tests/Drupal/Tests/Listeners/DeprecationListenerTrait.php
+++ b/core/tests/Drupal/Tests/Listeners/DeprecationListenerTrait.php
@@ -147,6 +147,7 @@ public static function getSkippedDeprecations() {
       'The "core/classList" asset library is deprecated in drupal:8.8.0 and is removed from drupal:9.0.0. Use the the native browser implementation instead. See https://www.drupal.org/node/3089511',
       'The "core/jquery.ui.datepicker" asset library is deprecated in drupal:8.8.0 and is removed from drupal:9.0.0. See https://www.drupal.org/node/3081864',
       'The "locale/drupal.locale.datepicker" asset library is deprecated in drupal:8.8.0 and is removed from drupal:9.0.0. See https://www.drupal.org/node/3081864',
+      'The "serialization.phpserialize" service is required for the bootstrap container. See: https://www.drupal.org/node/3014688',
     ];
   }