From what I know, most guides on piracy on steam deck involves running the game through Steam’s launcher as a “Non-Steam Game” which kinda feels sketchy to me, since the Steam Launcher portion of SteamOS is not open source, and they could be phoning home to Steam’s servers and reporting me as a pirate. Maybe they don’t go hard on pirates for now, but at any point in the future, they could pull a Nintendo move.
Steam deck works very well for retrogaming and for running non-steam games. You can set up emulators rather easily.
It’s an unlocked device (unlike a Nintendo), you run on it whatever OS you want. If they would pull such moves, community developed steam OS alternatives will arise. All that’s needed to run non-steam games on the device is open source.