11 files declare their use of FilterInterface in Drupal 8

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

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

Editor.php in core/modules/editor/src/Plugin/InPlaceEditor/Editor.php

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

EditorMediaDialog.php in core/modules/media/src/Form/EditorMediaDialog.php

FilterAPITest.php in core/modules/filter/tests/src/Kernel/FilterAPITest.php

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

FilterID.php in core/modules/filter/src/Plugin/migrate/process/FilterID.php

FilterSecurityTest.php in core/modules/filter/tests/src/Functional/FilterSecurityTest.php

Internal.php in core/modules/ckeditor/src/Plugin/CKEditorPlugin/Internal.php

ProcessedText.php in core/modules/filter/src/Element/ProcessedText.php

StandardTest.php in core/modules/editor/tests/src/Unit/EditorXssFilter/StandardTest.php