For punishing toxic behavior. I don't think riot needs to hide it. Let their automated system that alerts players, inform them specifically who and why is punished and do so regardless of if a report was sent in.
AFK teammate? Just leave lobby and move on. After a new game you get the Punishment popup." Player X was punished for Afking in your game."
Make it clear punishments are being delivered, rapidly, accurately, and maintain clarity that your player behavior systems are in place and properly targeting issues. Don't sweep it under the rug.
Its a PvP game. Players make a real impact on how enjoyable the games content is. Even 10% of the players being negative or toxic in a MMR range is a fucking MASSIVE amount of your games in that MMR. No joke, even a number as low as 10% shows up almost every match you play. Pick a number 1 to 10. Roll 9 D10. Any time you see that 10% number you have a "toxic" player. I ran a RNG test of 20 "matches" at 5% and the result was 12/20 had a Troll.
You got to aggressively combat ALL bad player behaviors and that's got to be a player behavior system MMR wont care long as it does not impact win %.