In short, you can of course play the Steam Deck offline. However, you can't always just disconnect and play a game offline straight after download: similar to playing games offline on Steam you often have to prepare the games for offline mode first. Additionally, some features such as multiplayer and cloud saves may not be available without an internet connection. So, it's always a good idea to check the requirements of specific games or apps to make sure they can be played offline.
The Steam Deck, similar to a PC, requires an internet connection for tasks such as downloading games and playing online multiplayer games. However, once a game is installed on the device and you've played it for a few minutes, it should be possible to play it while offline, unless the game specifically requires an internet connection. This feature makes the Steam Deck a versatile gaming device that can be enjoyed in various settings, even without a stable internet connection.