There's a lot of misinformation in the comments, if I can I'd like to clarify a few things:
Regarding games running without steam or online, and ruling out games that have online checks within the game itself (most games that do aren't single player):
Some games run without steam by default
A large percentage of games run without steam by providing the app ID in a file named steam_appid.txt in the game directory
Methods for running the vast majority of your purchased steam games without online and without steam exist but are outside this scope
These 3 categories shouldn't even matter if deck offline mode or the wifi switch are working correctly as of the recent SteamOS patch, and if they're not, file a bug report. For online checks within overly aggressive single player games, you'll need to look for an existing bypass. Shouldn't be much harder than troubleshooting games that don't work in proton by default. An exception for where there's no simple solution is denuvo games renewing every two weeks or so, can't renew without online.
I have had no trouble making any game worth playing on deck function assuming not borked on protondb, or incompatible anticheat. I even got a foreign game from the early 2000s with initially broken video playback working. If you want something to work on deck portably, it can usually be done.