First, I just want to be clear about this; are you suggesting that Riot give players the option to completely disable chat, or are you suggesting that Riot just remove chat outright?
Whatever the case, both are generally a disservice to the community as a whole, even if it does mitigate the abuse cases, and more than that, they don't actively prevent toxicity or help toxic players reform.
First, let's talk about abuse cases - people abusing the chat and just being toxic in general. It's a known that these people do eventually get their just desserts, but from moment to moment, these players still ruin games. And, of course, some people will think: "If people are just going to abuse the chat, why have it at all? Why not remove the chat if people aren't going to use it right?"
Well, removal of the chat only results in the removal of the abuse of chat. It doesn't affect players' toxicity, it only reduces the amount of ways they can express it. The next logical place: Pings. Without chat, toxic players would likely just turn to ping-spamming - Warning, Danger, and Enemy Missing pings.
Going on to the next logical extreme, what happens if you remove all non-gameplay channels of toxicity? No emotes, no chat, no pings? Well - then you get people resorting to gameplay toxicity. Trolling, griefing, intentional feeding, and so on. So, then what? The only options from then on is to remove the player, or remove the game.
You can't simply remove toxicity by removing the channels of expression toxic players use. It's sad to say, but the only method we have to combating toxicity is the methods we currently have in place - the Report System and the IFS.
Next, let's talk about reform - the biggest reason for the punishment system's ladder is to encourage reform. Each player gets roughly four shots at reform, sometimes more if they try, but still plenty of chances.
Without chat, without any avenue to really show that a toxic player has changed their behavior, we can't really see them reform - they can't show they've reformed. They can't show how well they can compose themselves without the chat, so a majority of the reform is kind of useless without it.
And, of course, the removal of chat would be a disservice to the playerbase at large, because it removes a majority of the interactions with other players. You can't have idle chatter without chat, and you can't make concise tactical observations/commands - the pings really only cover a scant few bits of information, after all.
- Neutral Ping (Can be used to target specific champions/minions/monsters/plants)
- Danger Ping
- Warning Ping (Very annoying, but usually more pronounced than the Danger Ping)
- On My Way Ping
- Enemy Missing Ping
- Assistance Ping (used for calling for aid from your teammates)
- Area Warded Ping
There's not much in the way of pings, and their uses are few and far between. Having chat allows you to communicate information with your teammates that can't otherwise be done through pings. Remember: Pings are only simple, short-hand communication tools, they can't totally replace chat communication.
And I've had numerous games that were made all the better from the chatter I had with teammates or enemies - particularly teammates. Even with people abusing chat, I would wholeheartedly advocate that chat stays, since the abusers will get dealt with anyhow.
All said, I can't see any real reason that the chat should ever be removed - and, in the case of allowing players to disable their chat, I don't think that'll happen, either. It can completely sabotage games where people are trying to communicate strategy, and one player is ultimately throwing the game because they can't see what's being said - all because they chose the lazy route of disabling chat instead of behaving better.