All hands on Steam Deck - Valve's handheld PC

[KevinC]

@Matt_W

No, the button is greyed out. You can transition to offline mode if you’re already online and lose connection while playing. You can’t go to offline mode if you wake up the Deck while already offline.

I disabled wifi so had no connection and was still able to launch games just fine. I’m not technically in offline mode, just no connectivity.