6 files declare their use of WebformMessageManagerInterface in Webform 6.x

RemotePostWebformHandler.php in src/Plugin/WebformHandler/RemotePostWebformHandler.php

webform.theme.template.inc in includes/webform.theme.template.inc

Preprocessors and helper functions to make theming easier.

WebformEntityReferenceLinkFormatter.php in src/Plugin/Field/FieldFormatter/WebformEntityReferenceLinkFormatter.php

WebformEntitySettingsGeneralForm.php in src/EntitySettings/WebformEntitySettingsGeneralForm.php

WebformMessageManagerTest.php in tests/src/Unit/WebformMessageManagerTest.php

WebformShareController.php in modules/webform_share/src/Controller/WebformShareController.php