On handhelds like the Steam Deck or ROG Ally, the lack of true offline play severely limits the "pick up and play" nature of the genre.
While you cannot be offline, you can simulate the feeling of a single-player game. Diablo IV is an MMO-lite, meaning you will see other players running around, fighting world bosses, and emoting in town. diablo iv offline mode
Dungeons and major campaign quest areas remain private to you (or your immediate party). On handhelds like the Steam Deck or ROG
The lack of an offline option remains a major point of contention for many players. Server Stability & Queues: fighting world bosses