Google has discontinued its branded set-top box Google ADT-3, which has been used for testing the latest releases of Android TV and Google TV for almost three years.
Note that for several months Google ADT-3 was almost impossible to purchase.
The ADT-3 is the third in a series of developer devices that Google is offering to test apps with Android TV updates. The Google ADT-3 debuted in January 2020 for $79 and was sold through Askey. The predecessor, ADT-2, was released about two years earlier at Google I/O 2018.
While the Google ADT-3 is no longer available, Android TV 13 and Google TV 13 Developer Previews are still only available for this device. For now, the only way out for them is the Android TV OS emulator in Android Studio.