path_shrunk = Path(path.vertices / [1, aspect_ratio], path.codes)?
(Actually the old code looks wrong -- it would modify path's vertices in places too)

Done. But I think I still need to copy the codes.

No, because path codes (and vertices) are explicitly immutable

    .. note::

        The vertices and codes arrays should be treated as
        immutable -- there are a number of optimizations and assumptions
        made up front in the constructor that will not change when the
        data changes.

so it's fine to reuse the same array.

I read this the other way round: "Don't modify vertices and codes." We assume that they stay the same, but it's not checked.

It's not checked but what happens if someone changes it is explicitly undefined behavior, so we can assume that they are not changed.

IMHO this claims that users are not allowed to change the code. But it does not guarantee that Path does not modify the input. In fact, if codes is a numpy array already, it's writable flag is set to False.

Anyway, I don't want to go into a discussion about who owns the data. In this place, it's just reusing codes from another path, so nothing bad should happen anyway. I've removed the copy, which is consistent with the current behavior (the slice codes[:] also shares the data).