A controversial subject
I don't think its right for Riot Games to permanently ban account for toxic chat. Cheating, continually throwing games, win trading, sure. But I really don't think that "toxic chat" should be something that gets an account taken away from you that most people spend money on.
Yes, I did have an account permabanned. Since then I have moderately reformed. I still get "toxic" sometimes, but nowadays it seems like toxicity, along with just about everything else, is a matter of opinion. For example, I was playing a game and my jungler came to gank my lane, failed it, died to my laner, and I said "Don't gank my lane anymore". That, to him, was a toxic statement and from there they tilted and became toxic themselves. From there things tend to escalate in most situations that I've seen and "ban worthy" things are said or done. But I don't think those things said should be "ban worthy".
My reason for my opinion is because there are countless other games that you play online in which people are as toxic as people on League and possibly more so. Every one of these games has the option to mute the player that is bothering you. From that point you should just move on. If they intentionally throw the game then let them and send out the report.
I just don't think that Riot should ban people for things that they say. Riot Games doesn't know what that specific person is going through when they say the things they do. I wish I was a little better at putting what my thoughts are into words, but the bottom line is that they shouldn't be able to ban people for what they say. In my opinion its stealing people's money.
Again, like I said, cheating, continually intentionally throwing games, win trading, etc. Fine, ban them. But we as players have the tools to silence toxic people in the one game we might ever have to interact with them in.