The feature was first implemented in Lib/sets.py (arising from PEP 218) in pure python. It was found to be useful and carried forward to the C implementation for the built-in type.  The feature is documented but not highlighted in the Library Reference, "Both set and frozenset support set to set comparisons."

You are correct that feature became less performant when the swap-bodies code was eliminated; however, the feature still has value so that a user can write "s in t" rather than "frozenset(s) in t".

Thank you for the suggestion, but I'm going to elect to leave the code as-is. There's no reason break other people's code and remove a feature that is intentional, guaranteed, tested, and sometimes useful in applications that have sets of frozensets.

If you don't like the feature, it is okay to not use it ;-)