The frozendict built-in type is now implemented in Python 3.15 (documentation). Multiple modules have already been updated to support this new type (json, pickle, pprint, etc.).
There is still an on-going work to support frozendict in more stdlib modules, and maybe also replace dict with frozendict where it is relevant. There is also an on-going work on optimizing frozendict.
You can now play with frozendict in the main branch. The future Python 3.15.0 alpha 7 release (2026-03-10) will include it.