Applying Unified Labels with user-defined permission should produce an error in unsupported platforms
according to this article https://docs.microsoft.com/en-us/azure/information-protection/configure-policy-migrate-labels it is possible to apply Unified Label with user-defined permissions when a preview release of AIP v2 client is installed. however, for Office on Mac/iOS/Android, the behaviour is that "Users are not prompted for custom permissions and no protection is applied ". this give a false sense of security to the users. instead of failing silently, an error should be displayed to the users
This is available in the latest clients.