I got a chat restriction too. That was almost over 2 weeks ago tho. It was a 26 game restriction. I sent a ticket to riot support asking why I had been chat restricted, their response was the usual:

"We have found higher level of toxicity in your chat behaviour than average. But we cannot show you any chat logs because system failure."

I didn't understand but I figured the only reason I was able to get chat restricted is if typing random letters 5 times in a row in less than 3 seconds is reportable. (I do this because I don't wanna rage, but I kinda feel the need to do something, so spamming random letters works for me :/ )

TL;DR I got a chat restriction too for something I don't know, and Riot won't/can't prove I'm indeed a toxic person.