Block Internet
You can block Internet access for specific games by enabling "Block internet" in the Game Menu. This can be useful for testing, or preventing certain types of data collection in games.
If you have several games you want to block Internet access for by default, you can create a Steam Collection called BlockInternet and add your desired games to this collection. When you launch these games through SteamTinkerLaunch, the Block Internet option will be enabled by default.
You can also edit the Game Config file with a text editor and set BLOCKINTERNET="1".
Some games will not work without Internet access. It is common for games with DRM such as Denuvo to require an Internet connection when launched for the first time on that PC or with a new version of Proton. Some games such as Origin games may require an active Internet connection at all times when playing.
A good way to quickly check if your game requires an Internet connection is the Steam Deck Compatibility check. At the very bottom of this check, Valve will note if this game requires Internet access.
If your game does not have a Steam Deck Compatibility rating, or if it is unsupported on Steam Deck, you may be able to find information around online about which games require Internet access.
Some functionality may be lost if an Internet connection is unavailable for your game. Other games may warn you that an Internet connection is required but in reality it may not be. Your mileage may vary when blocking Internet access for games.
SteamTinkerLaunch tries to block the internet access for the game by prepending unshare -n -r to the game launch command. If you have trouble with the -r parameter, please see this StackOverflow answer.
When testing unshare-net via bwrap, the test game could not find its data-files, so this is hardcoded for now - feel free to submit a PR to help improve Block Internet functionality.