12 files declare their use of FilterFormatInterface in Zircon Profile 8

CKEditorAdminTest.php in core/modules/ckeditor/src/Tests/CKEditorAdminTest.php

Contains \Drupal\ckeditor\Tests\CKEditorAdminTest.

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

Contains \Drupal\editor\EditorController.

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

Contains \Drupal\editor\EditorXssFilterInterface.

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

Contains \Drupal\filter\Controller\FilterController.

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

Contains \Drupal\filter\Entity\FilterFormat.

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

Contains \Drupal\editor_test\EditorXssFilter\Insecure.

MigrateFilterFormatTest.php in core/modules/filter/src/Tests/Migrate/d7/MigrateFilterFormatTest.php

Contains \Drupal\filter\Tests\Migrate\d7\MigrateFilterFormatTest.

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

Contains \Drupal\editor\EditorXssFilter\Standard.