Estonia. I've seen 1980s or 90s heaps of junk with heated seats. It's kind of a requirement when it gets to -33C in the winter. It's also very cheap for manufacturers to add, so a lot of them include it as standard equipment in northern countries.

It was more common than AC here until AC became standard equipment in cars.

USA here. It mostly depends on the regional climate, in my experience. If you’re buying a new car in Texas, it probably won’t have heated seats in the options package. If you’re buying one in Vermont, it would be a lot more likely to be available.