I suggest to close this issue. We are free to remove private functions in minor releases (3.9.x) without any warning. Private functions are excluded from the backward compatibility warranty.

For example: "Names prefixed by an underscore, such as _Py_InternalState, are private API that can change without notice even in patch releases."
https://docs.python.org/dev/c-api/stable.html#stable

In C, it's easy to call the decode() method of a string object and pass the expected encoding and error handler.