14 files declare their use of AccessResultReasonInterface in Drupal 8

AccessAwareRouter.php in core/lib/Drupal/Core/Routing/AccessAwareRouter.php

AccessResultTest.php in core/tests/Drupal/Tests/Core/Access/AccessResultTest.php

Contains \Drupal\Tests\Core\Access\AccessResultTest.

config_test_rest.module in core/modules/rest/tests/modules/config_test_rest/config_test_rest.module

Contains hook implementations for testing REST module.

EntityAccessChecker.php in core/modules/jsonapi/src/Access/EntityAccessChecker.php

EntityAccessDeniedHttpException.php in core/modules/jsonapi/src/Exception/EntityAccessDeniedHttpException.php

EntityResource.php in core/modules/rest/src/Plugin/rest/resource/EntityResource.php

FieldResolver.php in core/modules/jsonapi/src/Context/FieldResolver.php

FileUpload.php in core/modules/jsonapi/src/Controller/FileUpload.php

MediaLibraryAccessTest.php in core/modules/media_library/tests/src/Kernel/MediaLibraryAccessTest.php

RelationshipFieldAccess.php in core/modules/jsonapi/src/Access/RelationshipFieldAccess.php

ResourceResponseTestTrait.php in core/modules/jsonapi/tests/src/Functional/ResourceResponseTestTrait.php

ResourceTestBase.php in core/modules/jsonapi/tests/src/Functional/ResourceTestBase.php

UserAccessControlHandler.php in core/modules/user/src/UserAccessControlHandler.php

WorkspacePublisher.php in core/modules/workspaces/src/WorkspacePublisher.php