So I've been playing this game for over 4 years. New accounts, trying things out, picking off meta, trolling sometimes, picking braindead champions, picking meta champions, going Jarvan Camille botlane and dominating, and there is one thing I've noticed.
Every single game has somebody who doesn't care, win or lose, they remember its just a game. Then there are 2 people who care, but not enough to afk over a bad support pick. Then there are 2 people who either troll, type way to much, or get passive toxic in game.
And you know what, i want this to change, almost all players do. Nothing is going to change these things however.
But here is an idea.
Every time a player logs in, present them with their chat logs, ALL OF THEM, from the last time they logged in. Have these appear on screen, and don't allow the player to remove them or start a game until maybe 2-3 minutes go by. Many people are only toxic sometimes, or rarely, but some circumstances in game can bring it out. Maybe after seeing the things they've said, they can reform themselves before it gets too far.
Players getting out of hand ruins the game, toxic or whatever many players don't care about. Showing somebody the sheer amount of messages they've typed over a few games can be an eye opener to many players.