If you look to play the upcoming "Spectre Divide" game, don't hold your breath this is from an interview with Mountaintop Games CEO via The Verge.
It’s only currently planned for PC, with no controller or console plans yet — and Mountaintop won’t necessarily allow Steam Deck to join. “Steam Deck is a concern as a cheating vector, and I think our anti-cheat systems may block it right now,” Mountaintop CEO and cofounder Nate Mitchell tells me.
they didn't say using Steam Deck per se is cheating, but that it's a cheating vector, i.e. it's (according to them) easier to use cheats on it than on Windows PCs.