No, the workaround was for duplicating the existing behavior if the fix (raising an error like a normal dict does) broken someone's code.  The *only* possibility here is to have a __reversed__ that raises a TypeError.

What is it that makes reversed raise a typeerror on dict here?  Not that we can change it at this point, but reversed blindly using len and __getitem__ for user classes but not on dict is rather inconsistent.  I suppose the dict TypeError special case catches common mistakes?  In which case adding a __reversed__ that raises a TypeError to Mapping seems to make sense for the same reason.