Ive had mental health problems (Nothing major, just general depression and anxiety) and the toxicity bothered me a lot more when I started playing the game years ago. I dont think Ive become as desensitized as much as Ive realized that some people just unload emotions on the internet. While I used to feel more personally attacked by rage, I think they're just releasing it at whoever is currently in front of them.
I think there are too many players that dont understand that ranked is a marathon, not a sprint. If they arent winning every game, they arent seeing improvement in their rank. They see the top 10 players in challenger with 60%+ win rates, not realizing that's mainly due to the start of the season, placements, getting paired with some diamond players on their way back to challenger, and that even most challenger win rates will normalize to low/mid 50%. Most players who are improving at a normal rate usually have just over a 50% win rate (Like 50-55%) when playing vs players at near skill levels. So even players who are improving barely win more than half their games.
A lot of people just get angry at losing, and have problems dealing with those emotions. Partly due to being younger and less mature/experienced with it, and partly due to possibly deeper issues. Ive learned that it's not usually personal, even if it's targeted at me. 99% of the time I know they'll have forgotten about me by the time they're in their next game, and they'll just being yelling at someone else. I just mute and move on, as Ive realized there's no reason to let them batter me with verbal abuse (Im sure it would get to me if I allowed myself to read it all game, one statement Ive learned to shake off easily), and report them when the match is over.
This forum shows how frequently people are getting punished with how many people are posting about their punishments. It seems like a lot of people dont even realize they're being toxic until they post their chat logs here and have like 10 people tell them they deserve it. And Im sure the amount that post here are just a fraction of players who are being punished for it.
Unfortunately, there are millions and millions of players, so dealing with all toxicity is pretty much impossible, especially when it's a free to play game where people can just make another account (Granted they have to start back at lv 1, Im sure that gets old with consistent bans). The level of toxicity allowed is WAY less than it used to be. Years ago you could badger people with no punishment as long as you didnt use any racial/homophobic slurs. Now you get a chat ban (Escalating to bans if frequent enough) just for being negative towards someone in nearly any way.