To play games without the internet with Steam offline mode you must first prepare your Steam account for this, and then enable the feature.

How to Enable Offline Mode on Steam Deck By Carl Posted on January 6, 2023 Updated on August 28, 2023 Steam Deck allows Steam Deck users to play games in online and offline mode.

Yesterday it finally happened, thanks to the 1.1 patch for Steam, but only partially—very oddly, the offline mode is only available on Steam Deck.