13 files declare their use of ContentEntityBase in Zircon Profile 8

BlockContent.php in core/modules/block_content/src/Entity/BlockContent.php

Contains \Drupal\block_content\Entity\BlockContent.

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

Contains \Drupal\comment\Entity\Comment.

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

Contains \Drupal\entity_test\Entity\EntityTest.

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

Contains \Drupal\aggregator\Entity\Feed.

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

Contains \Drupal\file\Entity\File.

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

Contains \Drupal\aggregator\Entity\Item.

MenuLinkContent.php in core/modules/menu_link_content/src/Entity/MenuLinkContent.php

Contains \Drupal\menu_link_content\Entity\MenuLinkContent.

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

Contains \Drupal\contact\Entity\Message.

Node.php in core/modules/node/src/Entity/Node.php

Contains \Drupal\node\Entity\Node.

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

Contains \Drupal\language_test\Entity\NoLanguageEntityTest.

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

Contains \Drupal\shortcut\Entity\Shortcut.

Term.php in core/modules/taxonomy/src/Entity/Term.php

Contains \Drupal\taxonomy\Entity\Term.

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

Contains \Drupal\user\Entity\User.