11 files declare their use of AttachmentsInterface in Drupal 9

AjaxResponse.php in core/lib/Drupal/Core/Ajax/AjaxResponse.php

AjaxResponseAttachmentsProcessor.php in core/lib/Drupal/Core/Ajax/AjaxResponseAttachmentsProcessor.php

AttachmentsTestDomainObject.php in core/modules/system/tests/modules/early_rendering_controller_test/src/AttachmentsTestDomainObject.php

AttachmentsTestResponse.php in core/modules/system/tests/modules/early_rendering_controller_test/src/AttachmentsTestResponse.php

BigPipeResponseAttachmentsProcessor.php in core/modules/big_pipe/src/Render/BigPipeResponseAttachmentsProcessor.php

BigPipeResponseAttachmentsProcessorTest.php in core/modules/big_pipe/tests/src/Unit/Render/BigPipeResponseAttachmentsProcessorTest.php

BigPipeTestSubscriber.php in core/modules/big_pipe/tests/modules/big_pipe_test/src/EventSubscriber/BigPipeTestSubscriber.php

EarlyRenderingControllerWrapperSubscriber.php in core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php

theme.inc in core/includes/theme.inc

The theme system, which controls the output of Drupal.

Token.php in core/lib/Drupal/Core/Utility/Token.php

TwigExtension.php in core/lib/Drupal/Core/Template/TwigExtension.php