Fortunately in my case at least the unpleasantness was limited to the odd loony stalking me across the site to carry on arguments that were several days stale. I did get told to kill myself once, but the person making that suggestion sounded so unhinged their rant had very little weight! To be honest it wasn't abuse from other users that were affecting my mental health at all, it was the never-ending doomscroll of depressing content and the people who seemed almost gleeful at bad news because it validated their worldview (and this ironically seems to apply across the political spectrum). /r/ukpolitics prior to perhaps 2015 or so was something really special because people entertained ideas they disagreed with for the purpose of debate, rather than this ideological total war that is the norm on the internet now where any inch of retreat is cowardly and any notion that the "enemy's" arguments might have a point is treason. Of course it wasn't perfect (/pol/ raided it from time to time which would crapflood the whole sub with obvious bait), but the arguments were mostly good-faith debate rather than simply rhetorical cudgels to hit your opponent with.

That's what made me leave Reddit fundamentally, I wanted a debate rather than a conflict. I want people who'll tell me why I'm wrong, not people who'll call me an irredeemable piece of shit for not clicking my fingers and 100% agreeing with some philosophy neither of us really had an in-depth knowledge of. It's like internet debates (especially on Twitter) have adopted the old Calvinist notion of "total depravity" for the modern age. It's not even about being right, it's about other people being wrong and that'll make you an angry, bitter person if you're jumping headlong into that world for hours a day.