
It installs and during boot, Ubisoft Connect actually works. It would appear that Ubi actually made UbiConnect FINALLY work on Linux. Or some kind of compatibility was finally added. But finally boots with no tricks or secondary programs.

Apesar de que el juego corre de muy buena y se ejecuta bien mi mayor problema es que no se ejecuta el audio desde el inicio, no se escucha nada, ni la introducción del juego, ni el menú ni cuando estás en el juego, absolutamente nada
Usar otro protón hace que el juego ni siquiera se inicie (en mi caso).
uplay://launch/1875
Got it to work by following these steps:
- Switch to Steam Desktop mode
- Open Linux Discover App and install ProtonUp-Qt and Lutris
- Open ProtonUp-Q and add version GE-Proton8-32
- Open Lutris and run Ubisoft Connect app, login and install the game
- In Lutris sidebar click the person icon and log into Ubisoft connect again to show games in Lutris
- Right-click game in Lutris Games window and select Configure -> Runner options -> Wine version -> Proton 8.0
- Now run the game and see it will work!

En primer tienes que cambiar el lanzador de Ubisoft play que viene predeterminado por Ubisoft connect para poder iniciarlo y ejecutar
Realmente amaba este juego, pero un tiempo para acá dejo de ejecutar, cabe recalcar que remplaze Ubisoft play por Ubisoft connect y por un tiempo estuvo bien pero ahora solo inicia las imágenes que presenta quien es la desarrolladores y distribuidores pero antes de entrar al menú se cierra, probé toda versión de protón pero aún así no cambia,
El ejecutar no te garantiza que puedas a llegarlo a jugar, con el problema de que no llega ni al menú lo hace imposible de jugar

Changed Proton to GE-Proton-8, otherwise it got stuck on a black screen after nvidia logo (and ubisoft connect sync).
a lot of notes mention ubisoft play but i did not change anything and it tried with ubisoft connect anyways. had played on pc before so it synced saves properly. default video was set to low so i increased it to medium and just to be on the safer side limited fps to 40. no issues so far.

Tried to use Proton 9.0-1, 8.0-5, 7.0-6, 6.3-8, Experimental, and GE 9-2. None were able to boot into the game successfully. Proton 9.0-1 and GE 9-2 were able to get through the credits but then crashed after that.

Used Ninter's instructions from the AC2 page (https://www.protondb.com/app/33230#nuYTcGb5bn) to replace the Uplay launcher with the current Ubisoft Connect launcher.
Cutscene subtitles trend slightly on the miniscule side.
Settings used:
- 1280x720/90hz (setting to 800p stretches the image)
- Vsync - Off
- Environmental Quality - High
- Textures Quality - High
- Shadows Quality - High
- Ambient Occlusion - HBAO+
- Anti-Aliasing - MSAA2x + FXAA
Framerate is variable depending on location; at ground level, FPS fluctuates around the mid 30s, higher up is closer to the 40s. Cutscene framerate does dip somewhat into the high 20s, but mostly during character close-ups and effects-heavy shots. A 30hz lock is probably a good idea (I couldn't get a consistent 40 FPS without severely compromising the game's visuals).

Install game via Steam (don't launch it); Download Ubisoft Connect Installer; Replace UplayInstaller.exe with the Ubisoft Connect one in the /SteamLibrary/steamapps/common/Assassin's Creed Syndicate/Support/Software directory; Launch game and let Steam finish the installation.
Video lags and falls behind audio in cutsceens
Using Steam Remote Play with a sufficent system results in a perfect handheld playing experence.

Works fine once UplayInstaller.exe is replaced with UbisoftConnectInstaller.exe (downloaded from Ubisoft's website)
Uplay launcher no longer works, the file must be replaced with Ubisoft Connect.
Go to /steamapps/common/Assassin's Creed Syndicate/Support/Software/GameLauncher/
Delete UplayInstaller.exe
Download UbisoftConnectInstaller.exe, rename it to UplayInstaller.exe and paste it into /steamapps/common/Assassin's Creed Syndicate/Support/Software/GameLauncher/
The game should now launch with Ubisoft Connect instead of Uplay.

Tries to connect to uplay when ubisoft connect has superseeded it. When trying to log on in offline mode launcher crashes

I did not try offline mode cause i wanted to continue a save from my pc. i tried experimental, GE-Protons versions. and could not get it to update. not sure if offline mode works i never tried it. it is a pass on steam deck for me. hopefully we can find a fix somehow.

Used the following settings and the experience, through my short playtime yet, have been smooth: https://www.reddit.com/r/SteamDeck/comments/uuxir1/comment/i9hzq20/?utm_source=share&utm_medium=web2x&context=3
Uplay wouldn't update to Ubisoft Connect and would only suggest offline mode
Text is a bit small, including subtitles. Ensuring a 16:10 display ratio in the settings improves readability.
When first playing, all textures were missing and the screen flashed. Changing the settings to the ones suggested and restarting the game fixed it.
The main issue is the Ubisoft launcher not updating. This problem may also occur with Assassin's Creed Unity. In order to rectify this issue and being able to play the game, I followed the instructions present here: https://www.reddit.com/r/SteamDeck/comments/11kucn7/comment/jbn8vil/?utm_source=share&utm_medium=web2x&context=3
To reiterate, if you can manage to install the latest version of Ubisoft Connect, simply overwrite AC: Syndicate's Ubisoft Launcher. The gameid for Syndicate is 368500 .
In my case, as Unity's launcher updated correctly, I used that folder to overwrite Syndicate's and it worked like a charm, picked up DLCs and linked to the appropriate Ubisoft account.
The game crashed once and once the audio was weird after waking up from sleep mode