Honestly shadow chat bans are just a bad idea. Just ban them altogether for the same amount of time. Too much of this game is communication for it to make sense.
But if not then at the very least you need to inform all players in a game that the player is chat banned. Otherwise you get a lot of circumstances where players don’t know a player is banned and they get enraged because of it… especially because they player themself may not be aware no one can see their message.
If you want to see how this plays out, you can see it here. The orange player is shadow chat banned, and both they and the other players don’t know it. This leads to the black player raging about decisions the orange player is making. But the orange player is only making these decisions because they think they’re being ignored in the chat. So the black player rages and tries to throw the game.
So there are cascading affects to your decision to have shadow chat bans. It’s literally causing more toxic and cheating behavior rather than decreasing it.