Hi there. I know this is very late, but I'll put the explanation here for others! We're not directly supporting that route, Steam Deck via Itch. What I can say is that the steam_appid.txt file is what should stop the game attempting to connect to steam - if detected where the executable runs (i.e. in the same folder) and with the value inside it present. I'm not sure how that interacts with Proton, I'm afraid, but it might be worth looking at where Proton thinks it is running and ensuring the file is in the same place!
Haha, thanks for the reply! That was one of the first things I tried, but renaming steam_appid.txt didn't change anything. I decided to take another look just now, though; turns out what does it is the presence of Steam.exe in `C:\Program Files (x86)\Steam` under the Wine prefix, regardless of how you actually launch the game! Steam will always copy some stub binaries there if you use it to start a game with Proton.
Fortunately, the workaround is still pretty simple:
- If you've already launched your Steam shortcut at least once, delete the Wine prefix for it at `steamapps/compatdata/<id>/pfx` (Protontricks can do this quickly; deleting and recreating the shortcut probably also works).
- Set the launch options for the shortcut like so:
STEAM_COMPAT_CLIENT_INSTALL_PATH=/var/empty %command%
(Any old directory, or maybe any string at all, ought to work in place of `/var/empty`; the variable is supposed to point to your Steam directory, so Proton can find the Windows compatibility shims.) - That's it! The game will start and play normally now.
Amusingly, if you leave steam_appid.txt intact in the game's folder, Steam will pick up on it and appear to behave like you're playing the Steam version, with an entry for it in your library that says "Running" and everything! It doesn't magically give you a Steam copy or even change anything in the game itself at all (there are no achievements, it doesn't change your Steam profile, and the game's library page has a "Purchase" button instead of "Play"), but it is funny nonetheless. Steam is an intricate beast for sure.
Edit: So it turns out leaving the appid file has another side effect, but only in the Deck UI (and possibly standard Big Picture on other devices, I haven't checked): the controller uses the input scheme that is set for the Steam version of SOLAS, not the non-Steam shortcut. Unfortunately, if you don't own the game on Steam, Steam won't let you modify the controller config for that version (you can access the config screen in the Deck UI, but any change you try to make is silently discarded). Woops!