First, COD: Some COD titles appear to be working reasonably fine, some are not. And that’s offline, it will probably not work multiplayer. To be sure it will work you would probably have to install Windows on the deck. While that is very possible and a lot of people have done that because of the whole anticheat situation, it’s not what the deck was designed for. You will usually have significant worse performance on many titles on Windows and you will have to deal with running and OS designed for a mouse and keyboard on a handheld (though you could try to get around that with big picture mode).
Forza and GTA 5 appear to be working fine on the steam deck, though ive heard forza can sometimes crash randomly.
Would you care to share the name with us, fellow reddit explorers?