Step 1 — Install Assassin's Creed 2 on Steam. Chances are, you already did.


Step 2 — Install Protontricks. If you already have it, uninstall it and delete the data. Then reinstall it.


Step 3 — Open Protontricks and select Assassin's Creed 2.


Step 4 — Press install an application and Done. A pop-up should then appear.


Step 5 — Scroll down and find Ubisoft Connect.
Turn it on and press Done. It will slowly start the installer,
just hang on while it wraps things up.


Step 6 — If you verified Ubisoft Connect installed, close out of everything.
Manage the game through Steam and add the compatibility patch Proton Experimental. In my experience, it stopped all of the random crashes.


Congrats! The game should let you on now. You may have to change the controls in the game settings to Xbox 360 / generic to play with the Steam Deck.

VERY IMPORTANT DISCLAIMER!!! Unfortunately, you cannot get the Uplay rewards (like OG Altair's robes) anymore through the game itself. If you want them, use this save file below to start your journey instead. It has all of them and no story progress.
https://www.nexusmods.com/assassinscreedii/mods/23?tab=description

Remember to back up your save files first if you already made progress and want to keep it!!