16 files declare their use of ContentEntityBase in Drupal 9

Comment.php in core/modules/comment/src/Entity/Comment.php

ContentEntityBaseUnitTest.php in core/tests/Drupal/Tests/Core/Entity/ContentEntityBaseUnitTest.php

ContentModerationState.php in core/modules/content_moderation/src/Entity/ContentModerationState.php

EntityTest.php in core/modules/system/tests/modules/entity_test/src/Entity/EntityTest.php

EntityTestUpdate.php in core/modules/system/tests/modules/entity_test_update/src/Entity/EntityTestUpdate.php

EntityTestWithBundle.php in core/modules/system/tests/modules/entity_test/src/Entity/EntityTestWithBundle.php

Feed.php in core/modules/aggregator/src/Entity/Feed.php

File.php in core/modules/file/src/Entity/File.php

Item.php in core/modules/aggregator/src/Entity/Item.php

Message.php in core/modules/contact/src/Entity/Message.php

NoLanguageEntityTest.php in core/modules/language/tests/language_test/src/Entity/NoLanguageEntityTest.php

PathAlias.php in core/modules/path_alias/src/Entity/PathAlias.php

Shortcut.php in core/modules/shortcut/src/Entity/Shortcut.php

StringIdEntityTest.php in core/modules/migrate/tests/modules/migrate_entity_test/src/Entity/StringIdEntityTest.php

User.php in core/modules/user/src/Entity/User.php

Workspace.php in core/modules/workspaces/src/Entity/Workspace.php