> This looks like a backward incompatible change in 3.8.

Indeed. 

> Should not copytree convert arguments of the ignore callback to str and list correspondingly?

Well, since any existing code probably expects that behavior (or at least probably works if that is the case), I would be for such a change. I'm not sure what your policy is though. If there is recently written code that assumes the new types, are you OK with that breaking if it is changed back?

I guess since it's an undocumented breaking change, it shouldn't be too much of an issue.