Most subject-specific forums are actually ok. Because posting there demonstrates that you have something worthwhile to care about. Of course one can troll and flamebait on such forums as well but it takes effort and it's not going to seriously rile people up about anything. Twitter is poles apart from that, it's like being in a different universe.
Also, I noticed how most underdog / less socially-acceptable lifestyle/interest forums tend to be pleasant, humorous, and reasonable. The other aspect maybe that marginalized people (without chips on their shoulders resentment) know what it feels like to be othered / not treated well and go out-of-their-way to be friendlier. For example, I can't remember any LGBT+ people who aren't cool, decent, and sociable... and I'm the goofy, straight, ally interloper stealing all the pretty cis girls (or they're stealing me, IDK).
On niche interests-side where it's a small world, I think the cosiness reduced sized and inherent common interests also reinforce, promote better behavior, and friendliness.
Twitter and such definitely throw unbounded numbers of random people at each other, and so the odds of clashing are astronomically-higher. In this alternate (mainstream) universe, the sad part is that social and online ideological Balkanization has cemented echo chambers of memetic civil war; a people divided-and-conquered.
