Now that I have a Steam Deck, I’d like to bump this again. itch doesn’t even need to do anything fancy, just handle downloading and unpacking the games. I can do the rest of the setup from inside Steam myself.
I solved this for me by giving itch a wine executable to work with. Itch can download Windows games on Linux if a wine command can be found at a location defined in the PATH environment variable. I edited the itch desktop file to add the Steam Proton Experimental directory to PATH by replacing:
Exec=/home/deck/.itch/itch %U
with
Exec=env PATH="/home/deck/.local/share/Steam/steamapps/common/Proton - Experimental/files/bin/":$PATH /home/deck/.itch/itch %U
After restarting itch through the desktop shortcut, I can download Windows games and add them to Steam using the regular “Add a non-Steam game to library” process and selecting a Proton version in the compatibility properties.
Put your Steam Deck into Desktop mode by pressing the Steam button, clicking Power and select Switch to Desktop.
Open Firefox, download itch for Linux from https://itch.io/app and run the installer (double click itch-setup in the Downloads directory). Then right click on the itch icon that was created on your desktop and open with Kate, a text editor. There you replace the line that says:
Exec=/home/deck/.itch/itch %U
with a line that says:
Exec=env PATH="/home/deck/.local/share/Steam/steamapps/common/Proton - Experimental/files/bin/":$PATH /home/deck/.itch/itch %U
On the next start of the itch client in desktop mode, you can download and install Windows games.
After installing a Windows game from itch, you open the Steam library in Desktop mode, click on Add a Game on the bottom left corner and Add a Non-Steam Game. Click Browse… and go find the .exe of the game you just installed. They’re in the directory /home/deck/.config/itch/apps. Either paste or type this into the address line in the file explorer, or navigate to it from your Home directory: .config is a hidden directory, so you either show hidden files by pressing Ctrl+H on a keyboard or by clicking on the settings icon in the top right corner of the file explorer (click the arrow if it’s hidden because the window is to small) and clicking Show Hidden Files.
If anything is still unclear, just ask!