How do I mute every player in a game?
You can disable the player's bubble chat: https://developer.roblox.com/en-us/api-reference/function/Players/SetChatStyle
And then disable the chat button:
game.StarterGui:SetCoreGuiEnabled(Enum.CoreGuiType.Chat, false)
SetChatStyle doesn't work in game though and I've never tested setcoregui in game either, but it doesn't give any warnings on the wiki page about using it in game so I assume you might be able to. Only one way to find out.