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.
Just FYI: Devices totally can phone home even if they are offline. Modern spyware capitalism is basically malware. They can easily collect data, store it somewhere and send it to their server when you do go online sometimes in the future.