I am not sure that raising an error is the best option. We can just convert an integer subclass to an exact int using _PyLong_Copy().
I am not sure that converting to an exact int in low-level C API functions is the best option. In many cases we use only the content of the resulting object ignoring its type (when convert it to the C integer or float, to bytes array, to new instance of int subclass). Creating a new exact int is a waste of time.
This is why I withdrawn my patches and this issue is still open.