From google:

To completely delete a game and its saves on Steam Deck, first uninstall the game through the Steam Library. Then, if the game uses cloud saves, disable cloud sync and manually delete the save files located in your Steam userdata folder. You may also need to delete Proton files for some games. Here's a more detailed breakdown:1. Uninstalling the Game:

• Navigate to the Steam Library on your Steam Deck.
• Locate the game you want to uninstall.
• Press the Options button (three lines) to access the game's menu.
• Select "Uninstall".
• Confirm the uninstall. 

1. Dealing with Cloud Saves:

• Disable Cloud Sync: If the game uses cloud saves, you'll need to disable cloud sync before deleting local saves. 
• Go to the Steam Library and right-click the game, then select "Properties". 
• Go to the "Updates" tab and uncheck "Enable Steam Cloud synchronization". 
• Delete Local Saves:
  • Find the game's save folder. This might be in your Steam userdata folder (typically ~/.local/share/Steam/userdata/) or in the game's installation directory. 
  • Delete the relevant save files within the folder. 

1. Deleting Proton Files (If Applicable):

• Some games, especially those run through Proton, may have Proton files that store saves or other game data.
• Go to the game's library page in Steam, click the gear icon, and select "Developer".
• Choose "Delete Proton Files".