I’m literally a yankee, as in a lifelong resident of the area north of the Mason-Dixon line and I’ve visited several historical civil war sites over the decades because they’re among the inexpensive points of interest within several hours of here.

I think you’re both kind of right. Everything the previous comment said is correct IMO about it starting with a negative connotation and people in the south probably hating it. But modern online usage feels pretty respectful too. Even in a professional setting, if I were on a typical call and somebody from Europe referred to somebody thousands of km (or miles, lol) away on the other side of the US as one of the yankees/yanks, I don’t think it would even register as something I’d remember. (well NOW it will because of this comment, thanks lemmy! :D )

Plus more recently, those of us who do wild shit like pay attention to the outside world don’t exactly take offense to people insulting this fucked up country/government/culture/etc. We’re right here agreeing with you. So something like “yankee” doesn’t stand out much when you read somebody across the world write “fuck all USians” and you think to yourself “…I can see that. That’s fair.”

In Australia we call them "seppos". Short for "septics", which comes via rhyming slang from "yank" -> "tank" -> "septic tank".