American politics, at least, has been very partisan at times long before social media showed up. Much of 19th century was like that, even outside of the Civil War. That recent period you referenced, when politics was mostly private and considered embarrassing to bring up, is actually more abnormal than normal in that regard. IMO, it just indicates a period of time after major political issues get decisively resolved one way or the other, and a new issue hasn't come up yet (or, more likely, hasn't come to the boiling point yet).