I don't recall, but...

You can't modify any builtin modules. Freezing modules effectively makes them builtin from a user's perspective. There are plenty of modules that can't be modified:

>>> sys.builtin_module_names
('_abc', '_ast', '_codecs', '_collections', '_functools', '_imp', '_io', '_locale', '_operator', '_signal', '_sre', '_stat', '_string', '_symtable', '_thread', '_tokenize', '_tracemalloc', '_warnings', '_weakref', 'atexit', 'builtins', 'errno', 'faulthandler', 'gc', 'itertools', 'marshal', 'posix', 'pwd', 'sys', 'time', 'xxsubtype')

I don't see why adding a few more modules to that list would be a problem.

Was the objection to feezing *all* modules, not just the core ones?