I dunno, I'm southern, and you're right that sweet iced black tea is the default anywhere below the Mason-Dixon line. The only real difference you'll run into outside of niche places is Lipton vs tetley vs whatever industrial food supply had.

But you can get unsweetened almost anywhere, and Chinese-American places will almost always ask if you want sweetea just to be sure.

Above the line, when I've traveled into damn yankee land over the years, it's changed. Back as a kid driving through the Albanians Appalachians up to Ohio, and Pennsylvania, once you got into west Virginia, it was a coin flip what you'd get between sweet and not, and anywhere north of that, it was unsweetened iced tea.

Last trip I took, sweet iced was default even in Pittsburgh and Philadelphia, the few restaurants we went to.

I remember going to DC on a school trip in jr high though, and McDonald's didn't have tea at all in the one we went to. Baltimore during the same late eighties era, I only went a couple of times, but the fast food places didn't have tea on the menu.

Now, all of that could have just been a matter of not going to the right places, of course. But it's what I experienced.

Around here, in the Appalachians down to the foothills, good luck getting unsweetened iced tea. My wife is a damn yankee that likes her iced tea fairly strong, but only a tiny bit sweet. She calls the extra dark and sweet we have in this area tea syrup lol.