16 files declare their use of FilterFormatInterface in Drupal 9

CKEditorAdminTest.php in core/modules/ckeditor/tests/src/Functional/CKEditorAdminTest.php

editor.api.php in core/modules/editor/editor.api.php

Documentation for Text Editor API.

editor.module in core/modules/editor/editor.module

Adds bindings for client-side "text editors" to text formats.

EditorController.php in core/modules/editor/src/EditorController.php

EditorXssFilterInterface.php in core/modules/editor/src/EditorXssFilterInterface.php

editor_test.module in core/modules/editor/tests/modules/editor_test.module

Helper module for the Text Editor tests.

filter.module in core/modules/filter/filter.module

Framework for handling the filtering of content.

FilterController.php in core/modules/filter/src/Controller/FilterController.php

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

Insecure.php in core/modules/editor/tests/modules/src/EditorXssFilter/Insecure.php

MediaFilterController.php in core/modules/media/src/Controller/MediaFilterController.php

MigrateFilterFormatTest.php in core/modules/filter/tests/src/Kernel/Migrate/d6/MigrateFilterFormatTest.php

MigrateFilterFormatTest.php in core/modules/filter/tests/src/Kernel/Migrate/d7/MigrateFilterFormatTest.php

Standard.php in core/modules/editor/src/EditorXssFilter/Standard.php

TestMediaFilterController.php in core/modules/media/tests/modules/media_test_embed/src/Controller/TestMediaFilterController.php

user.module in core/modules/user/user.module

Enables the user registration and login system.