YOU CAN PLAY THIS GAME ON STEAM DECK OFFLINE BY FOLLOWING THESE STEPS BELOW:

To play offline, you must first launch the game with Proton Experimental and sign into Ubisoft Connect online. Don't link Steam or check "Remember Me" yet. Go through all the prompts until you get into the game, then quit via the game's main menu, all the way out of the game. Load the game again and when the Ubisoft Connect login screen comes up, go to settings and select the "Load UbiSoft Connect in offline mode". Now put your login info in and check "Remember Me". If it asks to link your Steam account, you should do it now.

Now completely turn the Wifi off on your Steam Deck. It should allow you to play the game in offline mode!