9 files declare their use of FilterInterface in Zircon Profile 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

Contains \Drupal\editor\Plugin\InPlaceEditor\Editor.

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

Contains \Drupal\editor\EditorController.

FilterAPITest.php in core/modules/filter/src/Tests/FilterAPITest.php

Contains \Drupal\filter\Tests\FilterAPITest.

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

Contains \Drupal\filter\Entity\FilterFormat.

FilterSecurityTest.php in core/modules/filter/src/Tests/FilterSecurityTest.php

Contains \Drupal\filter\Tests\FilterSecurityTest.

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

Contains \Drupal\ckeditor\Plugin\CKEditorPlugin\Internal.

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

Contains \Drupal\filter\Element\ProcessedText.

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

Contains \Drupal\Tests\editor\Unit\EditorXssFilter\StandardTest.