7 files declare their use of EntityHandlerInterface in Zircon Profile 8

BlockAccessControlHandler.php in core/modules/block/src/BlockAccessControlHandler.php

Contains \Drupal\block\BlockAccessControlHandler.

ContentTranslationHandler.php in core/modules/content_translation/src/ContentTranslationHandler.php

Contains \Drupal\content_translation\ContentTranslationHandler.

DefaultHtmlRouteProvider.php in core/lib/Drupal/Core/Entity/Routing/DefaultHtmlRouteProvider.php

Contains \Drupal\Core\Entity\Routing\DefaultHtmlRouteProvider.

EntityViewsData.php in core/modules/views/src/EntityViewsData.php

Contains \Drupal\views\EntityViewsData.

MenuLinkContentAccessControlHandler.php in core/modules/menu_link_content/src/MenuLinkContentAccessControlHandler.php

Contains \Drupal\menu_link_content\MenuLinkContentAccessControlHandler.

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

Contains \Drupal\node\NodeAccessControlHandler.

ShortcutAccessControlHandler.php in core/modules/shortcut/src/ShortcutAccessControlHandler.php

Contains \Drupal\shortcut\ShortcutAccessControlHandler.