Hey James,
I don't know if there's a modern way to force warnings to appear for API_TO_BE_DEPRECATED.
I think the purpose of soft deprecation is to avoid generating the build warnings that you want. So at least the answer to the "why" part of your question is that the API authors explicitly didn't want those warnings to appear yet.
There have been some broader discussions around customizing which warnings do and don't show; the situation you bring up here probably fits among them somewhere.