Stadia is something we don't really talk about here too much now, as Google has let it slide considerably from the original aim but it's still going and it seems Google still has some interesting plans for it.
There's an upcoming Google For Games Dev Summit on March 15, which includes Android and Stadia. For the Stadia service that runs on Debian Linux, Google has a few talks including "Play Testing Made Easy on Stadia", "Stadia Adventures in slow server code on Unity" and "Profiling on Stadia" but the perhaps bigger one is titled "How to write a Windows emulator for Linux from scratch?" that notes:
Detailed overview of the technology behind Google's solution for running unmodified Windows games on Stadia. This is a deep technical walkthrough of some of the core concepts with the goal to allow curious programmers to better understand such technologies and potentially to build their own.
We don't know yet if it will be open source but the bigger question perhaps is why they didn't go with all the existing code that has already proven itself? We are of course talking about Wine / Proton, DXVK and VKD3D-Proton which powers Windows games on Linux desktop and the Steam Deck. It's already shown to have great performance.
It's an obvious move for Stadia though to help increase the game library, the same reason Valve created Proton from Wine to bring more games to Linux systems. We'll be taking a look over the talk on March 15 and will make some notes for you.