Google's Stadia specs said that Windows, OS X, Android, and iOS would be supported--nothing about a Linux Stadia release for a specific distro.
They are talking about their Stadia client. I'm talking about actual games developed for Stadia, which uses Debian Linux to run them. I.e. Stadia games are all Linux games, but they run on the server. Once developer makes a game for Stadia, it basically just needs some trivial modifications to make a normal Linux version. So when they don't plan to do it (while in case of Larian they are in general positive about Linux releases), it does look like some exclusivity ban. I surely hope it's not though.