13 files declare their use of EntityWithPluginCollectionInterface in Drupal 9

Action.php in core/modules/system/src/Entity/Action.php

Block.php in core/modules/block/src/Entity/Block.php

ConfigEntityBase.php in core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php

ConfigEntityBaseWithPluginCollections.php in core/tests/Drupal/Tests/Core/Config/Entity/Fixtures/ConfigEntityBaseWithPluginCollections.php

ConfigEntityImportTest.php in core/modules/system/tests/src/Kernel/Entity/ConfigEntityImportTest.php

EntityDisplayFormBase.php in core/modules/field_ui/src/Form/EntityDisplayFormBase.php

EntityDisplayInterface.php in core/lib/Drupal/Core/Entity/Display/EntityDisplayInterface.php

FilterFormat.php in core/modules/filter/src/Entity/FilterFormat.php

ImageStyle.php in core/modules/image/src/Entity/ImageStyle.php

MediaType.php in core/modules/media/src/Entity/MediaType.php

RestResourceConfigInterface.php in core/modules/rest/src/RestResourceConfigInterface.php

SearchPage.php in core/modules/search/src/Entity/SearchPage.php

Workflow.php in core/modules/workflows/src/Entity/Workflow.php