The ADT-1 was never released to the public, but handed out to developers after Google I/O 2014 as a test device for the new Android TV platform. It didn't get much attention from Google past Android 5.0.2, which was its last official update. However, developers are an unrelenting bunch and if they have an Android device lying around, you'd bet that they'll try to cook up ROMs for it as long as that's remotely feasible. (Exhibit A.)
So a few XDA members were tired of seeing the ADT-1 get shunned from OTA updates and decided to take care of the situation. Forget about Android 5.1, they want the latest 'mallow on their TVs and so they've built a new ROM based on the PureNexus project.
In order to flash this ROM, you'll need a custom recovery for the ADT-1 like TWRP, the new PureNexus ROM for the ADT-1 that weighs about 182MB, a dedicated GApps package that won't crash on your device, and to follow the instructions on the XDA thread linked below. When you're done, you'll get an Android 6.0.1_r3 ROM with plenty of additions, fixes, and adjustments. If you ask Artem, this is the Christmas gift he was just hoping for.