On an Android TV device, do the following:
Go to Settings> Apps> Show System Apps> Show All Apps> Show System Apps and select "Chromecast Android Shell"
Go to Force Stop, Clear Cache and Clear Data
Restart Android TV and wait for it to boot up
Grab your phone, close the Stadia application, if it was running, open the application again, and among the "Choose where to play" devices you should see your Android TV
After following these steps: https://speedtest.vet/
Then you will most likely see something like this: https://vidmate.bid/
Tested by me and many users in the comments on Mi Box S, Mi Box (3), Nvidia Shield and many AndroidTV TV models
Try it yourself and let us know if you are lucky to be able to play already!
Hopefully Google will whitelist our beloved built-in CCU or release a necessary update for us to enjoy soon (but very soon, not soon 2020)