Why did we remove the assert or requirement that users of this have this checked already?

The check was duplicated between the caller and the assert (none needed it for other reasons - they were just doing it to satisfy this requirement). I didn't think the requirement made the API conceptually clearer or the implementation simpler - on the contrary, I thought it was preferable to gather all the checks into a single function.

Perhaps your question was "what happened to the proper error?". It's still there - an undefined result now indicates that an error should be reported.