Currently the deprecation item flag is mostly useless. It can be a deterrent from new users picking a service but there is no way for existing users of a deprecated item to realize they are using a deprecated item.
Allow the different API's to see the deprecated flag on a feature service and then display UI to notify clients of deprecated service usage. Bonus points if you add UX to navigate to the item page to read about the deprecation and hopefully find a replacement.
As it currently stands, the deprecation flag is not helping to remove the usage of deprecated items. Make deprecations useful please.