Suggestion to reduce toxicity: Get rid of promos
It seems alot of people get really salty when people troll during their promos. The fact that they are in their promos is also publicly displayed which doesnt help the player.
As many people know, promos and LP are just cosmetic indicators of the more important metric that represents your actual skill level which is mmr. Since it is merely cosmetic and seeing how so many people can get so salty when people troll in their promos then it surely cannot be something that is positive for player behaviour.
I have played other ranked games before (mostly 1v1) games and a couple of years ago I had to win like over 50 games to be promoted to the next division (e.g. gold to platinum) and that didnt really affect my motivation or many other people as well. Although the developers now added tiers in between the divisions to those games in more recent patches, the fact that they are 1v1 games and not team games will drastically reduce the impact on player behaviour issues.
Although doing away with promos altogether may seem too radical, it may help to reduce the amount of promo games for a start. For example, maybe only have promo games from bronze 3 to bronze 2 and bronze 1 to silver 5 only for example (and same for silver and above up to gold or platinum) to reduce the general saltiiness. I have seen ample evidence that suggests people in lower elo cant handle promos that well anyway so it would be better to phase them out of low elo altogether and start having promo games at maybe gold onwards