Worth mentioning: Android TV is proprietary and developed by Google. As far as privacy goes it is as bad as any wish.com-android-solution.
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