6 files declare their use of CacheableResponse in Zircon Profile 8

DynamicPageCacheTestController.php in core/modules/dynamic_page_cache/tests/dynamic_page_cache_test/src/DynamicPageCacheTestController.php

Contains \Drupal\dynamic_page_cache_test\DynamicPageCacheTestController.

Feed.php in core/modules/views/src/Plugin/views/display/Feed.php

Contains \Drupal\views\Plugin\views\display\Feed.

PageCacheAcceptHeaderController.php in core/modules/system/tests/modules/system_test/src/Controller/PageCacheAcceptHeaderController.php

Contains \Drupal\system_test\Controller\PageCacheAcceptHeaderController.

RestExport.php in core/modules/rest/src/Plugin/views/display/RestExport.php

Contains \Drupal\rest\Plugin\views\display\RestExport.

SystemTestController.php in core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php

Contains \Drupal\system_test\Controller\SystemTestController.

TestControllers.php in core/modules/system/tests/modules/router_test_directory/src/TestControllers.php

Contains \Drupal\router_test\TestControllers.