> It is very little effort to add back the old function, so that isn't the problem. It won't work properly, but it never did anyway. So I guess that's sort of compatible.

It won't work properly is an incompatible change. Before, if you extract all fields from a code object and pass it down to the constructor, everything will work.

> Maybe the best thing is to put a big red warning in the docs and hope that warns away people from using it?

I think code object constructors must be part of the private CAPI due to what we are experiencing. But again, this is something we cannot decide on this bpo issue. Either a python-dev thread needs to be open or a Steering Council request in the https://github.com/python/steering-council repo needs to be opened.