I don’t think it’s the contentiousness of the topics, but the polarization of topics at issue. I’ve personally seen many complex/incendiary topics discussed with respect in situations where nuance was still possible. Unfortunately, especially this year, the number of non-polarizing topics has rapidly diminished.
When folks decide that their position is so self-evident that disagreement is either ignorance or malice, and opposing viewpoints have nothing to contribute, and worse, that there is only one opposing viewpoint in the first place, it becomes very, very difficult to try and foster discussions in that environment.
In tight-knit communities, I’ve seen the behaviour applied to someone who posts an unpopular view range from “ignore the comment” on the light side (or possibly noncomittal affirmation of their post) all the way to the “I knew this person wasn’t really our type” discussions in back-channels.
In more public communities though, this is especially difficult. A particularly polarizing post may well draw in new posters on either side of the debate, and depending on the dominant position of the established community, this usually leads to vitriolic or abusive retaliation on the part of “regular members” against the newcomers. Worse, because the newcomers are likely unaware of the nuances and culture of the community they are trying to join, the likelihood that they will make a mistake and get moderated for it is extremely high - doubly so if their experience until now has been on non-discourse forums with lax moderation or community cohesion.
I’m not sure what the solution is. “Slow mode” definitely helps, I’ve seen the effect already in the brief trials we’ve had of that feature - on contentious posts, the number of replies remains high while the number of contributors overall increases, but this also increases moderator workload to 1) identify this mode needs to be enabled for a given post quickly enough to keep it from derailing, and 2) to aggressively moderate the responses that do come in and/or post themselves to set the tone for the discussion. But this is just a bandaid to help with the symptoms of a larger issue - how do you encourage and support nuanced discussions on polarizing topics?