TvOs beta despite un-enrolling

Interesting. I'm not sure how you got the tvOS beta installed if you didn't enable the "Get Beta Updates" option. At any rate, it isn't possible to downgrade an ATV 4K so you're just going to have to move forward with the betas until the GM tvOS 16.2 is released.

To do a default reset on an Apple TV 4K follow these instructions: Reset or restore your Apple TV to its factory settings - Apple Support. I do still suggest this option that may restore the "Get Beta Updates" option for you in Settings > System > Software Update. Once the GM tvOS 16.2 is released you can then disable that option and not receive the betas anymore.

If you do not see the "Get Beta Updates" button, I've seen this before and what I had to do was do a default reset on the ATV device. I suggest that is what might solve your problem in the short term.

In the long term, since your ATV is already running a beta version, I'd suggest after the default reset you stay in the beta program so you keep getting updates up to the release of the GM. After you get the GM of 16.2 you can set the "Get Beta Updates" to OFF>

In the Settings app, go to System > Software Update. Set the "Get Beta Updates" button to OFF.

What version of tvOS is your device running right now?