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).
You will find the source code of Android, you won’t find the source code of Android TV.
Just like all the Android-based operating systems on smartphones: if you take a free product and add suckyness, the end product sucks.
If there’s no source code for ATV, how can there be (unofficial) LineageOS builds of ATV? Genuine question
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.
