Mute Notification
It seems to me that when I mute someone they should receive a mute notification informing them that I have them muted. There are a lot of benefits this would have.
First off, Riot has stated that reform rather than punishment is a high priority. If you want people to improve their behavior the instant feedback would be one of the most effective ways. As an example, when driving down the road, one of the most effective ways to get people to slow down is to show them their speed in comparison to the speed limit. In the same way, people who receive mute notifications would know that people just don't want to listen to them.
Second of all, players who are toxic would (hopefully) stop bothering to type out toxic messages berating people who have muted them. They will probably still complain to the rest of the team, but if I can get that person to reduce their typing to 50% of what it was, the will have their hands on the controls instead of typing on the keyboard, hopefully improving their performance.
I also would also like to know if I have been muted. I try really hard to be as positive as possible, so I don't think I get muted much, but I know that there are others who just mute everyone immediately. I'd hate to waste time messaging people who won't hear me.
Maybe this has been discussed before but I couldn't find anything and I think there are a lot of potential benefits to this kind of feedback.