Can the Demo be played on Steam Deck?
The official October 19 announcement says the Demo can be played on Steam Deck. The Demo FAQ also says it is not officially verified.
Those statements describe different things. A playable Demo announcement does not establish a Valve Verified rating, a stable frame rate or a set of best graphics settings.
Is there a native Linux version?
In a September developer reply, there were no plans for native Linux support. Steam Deck playability does not mean a native Linux build has been announced.
Plan around the Demo’s save rules
The Demo has no save system. Quitting means restarting the next session, and Demo progress does not transfer to the full game. See Demo access and saving before starting a longer session.