10 files declare their use of WebformHandlerInterface in Webform 6.x

Webform.php in src/Entity/Webform.php

WebformEntityHandlersForm.php in src/WebformEntityHandlersForm.php

WebformEntitySettingsGeneralForm.php in src/EntitySettings/WebformEntitySettingsGeneralForm.php

WebformHandler.php in src/Annotation/WebformHandler.php

WebformHandlerFormBase.php in src/Form/WebformHandlerFormBase.php

WebformInterface.php in src/WebformInterface.php

WebformOptionsLimitHandlerInterface.php in modules/webform_options_limit/src/Plugin/WebformOptionsLimitHandlerInterface.php

WebformPluginHandlerController.php in src/Controller/WebformPluginHandlerController.php

WebformSubmissionForm.php in src/WebformSubmissionForm.php

webform_test_handler_invoke_alter.module in tests/modules/webform_test_handler_invoke_alter/webform_test_handler_invoke_alter.module

Support module for webform that provides handler invoke alter tests.