Good point, and Łukasz Langa mentioned this in his talk (check it out). He names it the robustness principle, in his words (around 22:20 mark:

“Vague in what you accept, concrete in what you return”

But he also mentions some gotchas like how Iterable[str] can backfire, because str is also an Iterable[str] and it might be better to use list[str].