During Aang and Korra's times I don't think they existed, although in the real world they've been around as far back as the 1840s. But I randomly thought: There a chance an Avatar could get arbotioned, it would probably slow down a lot of things as people would keep looking for the avatar after their death, also if a situation like the one after Kuruk's death happened where people didn't know who the avatar was for years do you think they'd go onto the next nation and leave the unknown avatar to live a normal life?