Riot, you need to factor pre and post-match chat into toxicity and punishments. Here is why.
Behold my recent normals game. Our Ahri stole our jungler's first camp, flashed to lane, fed 6 kills in a row to Ezreal in 10 minutes, then disconnected, all without saying a word.
That's not the issue, Ahri will face justice with the tribunal.
But the Ezreal, as you can see, explicitly abuses the fact that Riot doesn't punish anything for what chat happens after the game and starts throwing around insults like "faggots" and brags about beating an intentional feeder.
He shows his extreme toxicity but he'll never be punished unless you change this backwards policy. Please punish players who troll champ select or insult in the end game screen while pretending to be good players in game.
