[Serhiy]
> In particularly it allows to modify __dict__ of builtin types.

This doesn't look like something that would happen accidentally.  We've never had any bug reports about this occurring in the wild.


[GvR]
> Maybe we should not fix this then?

That seems reasonable.  In general, it is hard to completely wall-off instances against deliberate efforts to pry them open:

    >>> from types import MappingProxyType
    >>> import gc
    >>> orig = {1: 2}
    >>> proxy = MappingProxyType(orig)
    >>> refs = gc.get_referents(proxy)
    >>> refs
    [{1: 2}]
    >>> refs[0][1] = 3
    >>> orig
    {1: 3}