I'm removing the tuple feature from this PEP. So now, if I understand
correctly, I don't think there's disagreement about behavior, just about
how that behavior should be summarized in Python code.
[...]
    return (the original object unchanged, or a copy of the object,
            depending on implementation details,
            but always make a copy when working with subclasses)

is well-summarized by

   return self[:]

especially if followed by the text

    Note that ``self[:]`` might not actually make a copy -- if the affix
    is empty or not found, and if ``type(self) is str``, then these methods

    may, but are not required to, make the optimization of returning ``self``.
    However, when called on instances of subclasses of ``str``, these
    methods should return base ``str`` objects, not ``self``.

[...]