Here's question to focus on:  In what circumstances should a developer ever prefer PyMapping_Check() over PyType_HasFeature() with Py_TPFLAGS_MAPPING?

The latter doesn't give any new, useful information:

   return o && Py_TYPE(o)->tp_as_mapping &&
               Py_TYPE(o)->tp_as_mapping->mp_subscript;

I don't see any reason to build on top of this. It's best to just let it go gently into the good night without disrupting anything that currently happens to work.