The game Warframe uses IRC for its in-game chat. They've obfuscated the authentication though, so it's not possible to just login with something like irssi. I'm sure other games do this as well.
Osu! also uses IRC for it's game chat, but it's not obfuscated . IRC is good enough for game chat, you don't really need anything more, unless you want to integrate voice-comms too. Actually, I once made multiplayer-over-IRC in a game jam, as the interface is so simple and flexible.