An article in The Verge yesterday covered the release of Google's new Chromecast with Google TV dongle, a replacement to the Chromecast Ultra device which is no longer being sold individually. It also revealed a curious shortcoming in the new device: it doesn't come with game streaming service Google Stadia. Google reportedly doesn't support Stadia at the present time, but they're expecting to get support added in sometime in the first half of 2021.
According to the article, Stadia can be loaded onto the Chromecast with Google TV via sideloading, but it doesn't seem to be officially supported. Moreover, it doesn't seem to work quite as well. The Verge reported in a separate piece that the best-case scenario is a wireless router next to the TV, which apparently puts the TV into low-latency mode, but attempting to purchase games in the Stadia app causes it to crash. They also reported longer response times when trying to load a large games library.
There is not any official explanation for the lack of support, but one theory advanced by The Verge centers around the difference between the Cast functionality in the Chromecast Ultra and the Android TV "OS" inside the Chromecast with Google TV device. The Chromecast with Google TV device doesn't just have Android TV, but also a Google TV "skin" which adds an extra layer of complexity to the equation. Stadia apparently doesn't play well with the Android TV system, and the Google TV layer potentially takes up resources which Stadia needs to run effectively.
Google's promise that they'll get support added in is encouraging for Stadia users, but the lack of getting it in place before launching the new device is questionable. Would-be Stadia subscribers can pick up a Chromecast Ultra as part of the Stadia Premiere Edition kit while waiting for Google to get things fixed.