18 files declare their use of Action in Drupal 8

ActionFormAjaxTest.php in core/modules/action/tests/src/FunctionalJavascript/ActionFormAjaxTest.php

ActionResourceTestBase.php in core/modules/system/tests/src/Functional/Rest/ActionResourceTestBase.php

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

ActionTest.php in core/modules/system/tests/src/Kernel/Action/ActionTest.php

CommentActionsTest.php in core/modules/comment/tests/src/Functional/CommentActionsTest.php

ConfigEntityImportTest.php in core/modules/system/tests/src/Kernel/Entity/ConfigEntityImportTest.php

ConfigInstallProfileOverrideTest.php in core/modules/config/tests/src/Functional/ConfigInstallProfileOverrideTest.php

ConfigurationTest.php in core/modules/action/tests/src/Functional/ConfigurationTest.php

DeleteActionTest.php in core/tests/Drupal/KernelTests/Core/Action/DeleteActionTest.php

MigrateActionsTest.php in core/modules/action/tests/src/Kernel/Migrate/d6/MigrateActionsTest.php

MigrateActionsTest.php in core/modules/action/tests/src/Kernel/Migrate/d7/MigrateActionsTest.php

MoveActionsToCoreTest.php in core/modules/system/tests/src/Functional/Update/MoveActionsToCoreTest.php

NodeActionsConfigurationTest.php in core/modules/node/tests/src/Functional/NodeActionsConfigurationTest.php

PublishActionTest.php in core/tests/Drupal/KernelTests/Core/Action/PublishActionTest.php

SaveActionTest.php in core/tests/Drupal/KernelTests/Core/Action/SaveActionTest.php

UnpublishByKeywordActionTest.php in core/modules/node/tests/src/Kernel/Action/UnpublishByKeywordActionTest.php

UpdateActionsWithEntityPluginsTest.php in core/modules/system/tests/src/Functional/Update/UpdateActionsWithEntityPluginsTest.php

user.module in core/modules/user/user.module

Enables the user registration and login system.