You can troll, go afk, throw games, feed, do the worst things you can imagine, just don't talk in chat and you will be fine. This is the policy of Riot for LoL, a game they changed for the new snowflake generation. They are going the SJW route with this game and it will go down faster and faster. They already lost so many players and will continue to lose more.
If you still want to use the chat, two things matter: the number of messages you write and the number of reports you get, nothing more.
I had people wishing me cancer or to die in different awful ways, kill myself etc. They still play the game, no ban or anything like this. But after 2 games with idiots, i got banned 14 days for chatting. In one game i told a "teemo main" to play something else, but he and his voli friend completely ruined the game. In my 2nd game our adc went afk ~20min. She came back after min25 or so, we had nothing left, but me saying that in chat made them report me. I always try to win my games, i never feed or play to lose. I'M ALWAYS TRYING TO WIN, but for talking i got banned 14 days now. Banned, not muted or anything like that. I didn't tell them to die, kill themselfs etc. They can also mute me in-game, but i cannot stop them from trolling, feeding, go afk etc.
All that matters for Riot is to have a nice friendly chat(this looks good for parents i guess). Go ruin other people games, but don't talk with them and everything will be fine. Riot chose this simple punishing system which is so bad and with so many problems; but easy to implement i guess and it makes them look good for this snowflake generation that gets offended from anything, while they act like complete a-holes.