Hi All,
I have recently managed to get this game working on my Steam Deck!
Here are the steps I took to get the game working:

1. Install Assetto Corsa via the SteamOS gaming mode.

2. Switch over to desktop mode.

3. Install the software called "ProtonUp-Qt" (This tool lets you to install custom Protons and Wine compatibility layers)

4.While in desktop mode, open up ProtonUp-Qt then find and install "GE-Proton9-11"

5. Once installed switch back to SteamOS gaming mode.

6. Find the Game Assetto Corsa in your library and force a specific compatibility tool selecting the "GE-Proton9-11".

7. Launch the game.... the first boot can take about 10 to 20 minutes, so be patient.

8. Once game has loaded, close it.

9. Re open the game... the boot time should now only be about 1 to 2 minutes before the intro movie starts.

10. Enjoy the game!

Note: Since the game isn't fully supported you'll need to adjust your controller layout. Either build your preferred layout in the Steam controller settings or download a community layout.

Hope this helps and I look forward to seeing you out on the track.
Cheers