Recently got this to work on steam deck following these steps:
1) Download
2) Extract and copy files to Valheim game folder
3) Open Terminal from Valheim game folder
4) Enter the following commands into terminal (minus the quotations):
- "cd .." – the first command will take you out of the Valheim directory into the directory called 'common'
- "sudo chown -R deck:deck Valheim" – the second will make all the files inside the Valheim directory be owned by the user called 'deck' and the group called 'deck'
- "sudo chmod -R 777 Valheim" – third command will change the permissions on all the files inside the Valheim directory to have read+write+execute
5) after that you open steam and go into Valheim's properties and you set the launch options to
./start_game_bepinex.sh ; echo %command%
Source: https://www.reddit.com/r/valheim/comments/107znsh/valheim_plus_on_steam_deck/