When did eskimo become a slur and why?
Because calling any of the "first people" or "indigenous" people who are not Eskimos, Eskimos, is pretty rude if you're not one.
Calling every one of that diverse group of people "Eskimo" is not only lazy and incorrect, it's pretty dismissive. There are a bunch of tribes and different groups, Eskimos are mostly the folks living right at the very top of the polar regions.
Imagine if every black person you met you called an Aborigine? Wrong continent, wrong region, wrong description.