There were exceptions. There was one forum I was a part of where none of us knew each other in real life and yet we had long, intense, good-faith discussions on touchy topics that stayed meaningful.

I think what made it different was that we were a tight-knit group that was invested in our pre-existing relationship: we started as a clan in a very cooperative video game and we had each other's backs every day in-game. The political and religious discussions occurred in our private forum in the off-topic section.

So I suspect you're half right: the conversations have to occur with people you trust and respect. But at least in the early 2000s, they didn't have to occur in-person.

What matters seems to be not that they be in-person but that they be private and that all involved cherish their relationships with each other. A lot of what happened in the last decade is that people have left the private spaces and moved into public spaces, where such conversations really do become toxic quickly. Even on Facebook with posts set to friends-only, the system encourages adding so many "friends" that it may as well be public. By the time most people get to friend #300, neither of them really cares about the relationship, so meaningful discussion about touchy topics becomes impossible.