12 files declare their use of ContentEntityTypeInterface in Zircon Profile 8

CommentStorageSchema.php in core/modules/comment/src/CommentStorageSchema.php

Contains \Drupal\comment\CommentStorageSchema.

ContentLanguageSettingsForm.php in core/modules/language/src/Form/ContentLanguageSettingsForm.php

Contains \Drupal\language\Form\ContentLanguageSettingsForm.

content_translation.admin.inc in core/modules/content_translation/content_translation.admin.inc

The content translation administration forms.

DefaultTableMapping.php in core/lib/Drupal/Core/Entity/Sql/DefaultTableMapping.php

Contains \Drupal\Core\Entity\Sql\DefaultTableMapping.

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

Contains \Drupal\Tests\Core\Entity\EntityFieldManagerTest.

EntityTestStorageSchema.php in core/modules/system/tests/modules/entity_test/src/EntityTestStorageSchema.php

Contains \Drupal\entity_test\EntityTestStorageSchema.

NodeStorageSchema.php in core/modules/node/src/NodeStorageSchema.php

Contains \Drupal\node\NodeStorageSchema.

RelationLinkManager.php in core/modules/rest/src/LinkManager/RelationLinkManager.php

Contains \Drupal\rest\LinkManager\RelationLinkManager.

SqlContentEntityStorageSchema.php in core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorageSchema.php

Contains \Drupal\Core\Entity\Sql\SqlContentEntityStorageSchema.

SqlContentEntityStorageSchemaTest.php in core/tests/Drupal/Tests/Core/Entity/Sql/SqlContentEntityStorageSchemaTest.php

Contains \Drupal\Tests\Core\Entity\Sql\SqlContentEntityStorageSchemaTest.

TermStorageSchema.php in core/modules/taxonomy/src/TermStorageSchema.php

Contains \Drupal\taxonomy\TermStorageSchema.

UserStorageSchema.php in core/modules/user/src/UserStorageSchema.php

Contains \Drupal\user\UserStorageSchema.