The proposed change will affect many builtin types: bytes, bytearray, str, but also other types like collections.UserString. Would it make sense to summarize what has been said in the python-ideas thread into a PEP? It may good to specify things like:

    >>> x = "A"*10**6
    >>> x.cutprefix("B") is x
    True

The specification can be just "that's an implementation detail" or "CPython implementation specific" :-)

I don't expect such PEP to be long nor controversial, but it may help to write it down.