11 files declare their use of CacheableResponse in Drupal 9

DefaultExceptionSubscriber.php in core/modules/serialization/src/EventSubscriber/DefaultExceptionSubscriber.php

DefaultFormatTestController.php in core/modules/system/tests/modules/default_format_test/src/DefaultFormatTestController.php

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

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

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

ResourceResponseSubscriber.php in core/modules/jsonapi/src/EventSubscriber/ResourceResponseSubscriber.php

ResourceResponseSubscriber.php in core/modules/rest/src/EventSubscriber/ResourceResponseSubscriber.php

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

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

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

VocabularyResponse.php in core/modules/taxonomy/tests/modules/vocabulary_serialization_test/src/VocabularyResponse.php