I think this approach is probably the best we can do, but I could also imagine that users might find it to be confusing behavior. I wonder if there's any informal user testing we can do?

I guess the ISO 8601 spec does call "Z" the "UTC designator", so `use_utc_designator` seems like approximately the right name. My main hesitation with this name is that I suspect users may think that `use_utc_designator` means that they *unconditionally* want to use `Z` — without reading the documentation (which we can assume 99% of users won't do) — you might assume that `dt.isoformat(use_utc_designator=True)` would translate to `dt.astimezone(timezone.utc).replace(tzinfo=None).isoformat() + "Z"`.

A name like `utc_as_z` is definitely less... elegant, but conveys the concept a bit more clearly. Would be worth throwing it to a poll or something before merging.