7 files declare their use of SerializationInterface in Drupal 9

DatabaseStorage.php in core/lib/Drupal/Core/KeyValueStore/DatabaseStorage.php

DatabaseStorageExpirable.php in core/lib/Drupal/Core/KeyValueStore/DatabaseStorageExpirable.php

FrontMatter.php in core/lib/Drupal/Component/FrontMatter/FrontMatter.php

KeyValueDatabaseExpirableFactory.php in core/lib/Drupal/Core/KeyValueStore/KeyValueDatabaseExpirableFactory.php

KeyValueDatabaseFactory.php in core/lib/Drupal/Core/KeyValueStore/KeyValueDatabaseFactory.php

YamlTest.php in core/tests/Drupal/Tests/Core/Serialization/YamlTest.php

YamlTest.php in core/tests/Drupal/Tests/Component/Serialization/YamlTest.php