It might still be possible to change, by changing the macros that unwrap integers to support tagged integers, then waiting a decade or two before actually starting to use tagged integers. Things that hadn't been recompiled in a decade or two, but were for some reason running the latest interpreter would break. Presumably the number of breakages would be negligible.
Sure, but the only opportunity to change this was along with the other breaking changes in Python 3.0, which is now 12 years old.