To my knowledge Android TV isn't any more proprietary than standard Android. The only really proprietary code are the Google Play Services (and Widevine).
Do you mean this konstaKang-Image? LineageOS for the RaspberryPi, I tried that and it works quite ok. A bit sluggish maybe.
LineageOS is an open source project build around the pure open source Android. And the konstaKang-Image is also an open source project, although with a weird license. But they have nothing to do with Android TV, as far as I know.