It seems for the most part that there some purists who want everything to be annotated with #[must_use], and that's certainly another discussion and an option for those people is to just turn on a lint for every unused result, but doesn't seem to be tackling the question of whether this specific function should be tagged with #[must_use].
I don't see any good arguments why it shouldn't be marked as #[must_use], unless the map function is performing a side effect, but that is not the intended purpose of map, and likely that case is fine to ignore.
Given the replies thus far, I'll just submit a PR.