There's a lot of changes going on in CPython in the past few releases, so a frozendict wouldn't be a big one comparatively speaking. The bigger problem is that

There's a lot of changes going on in CPython in the past few releases, so a frozendict wouldn't be a big one comparatively speaking. The bigger problem is that there was already a PEP (https://peps.python.org/pep-0416/ https://peps.python.org/pep-0416/) about that, and it was rejected for wonderful reasons such as "multiple threads can agree by convention not to mutate a shared dict, there’s no great need for enforcement" and "there are existing idioms for avoiding mutable default values".