Stop. Breathe. Let it out slow. Yes, iOS 12 Beta was experiencing an issue where it continuously popped up spurious notifications saying "A new iOS update is now available. Please update from the iOS 12 beta"/. And yes, it's super annoying.
Well, Apple has just pushed out a fix. So, go to Settings > General > Software Updated, download and install it, and say goodbye to those annoying popups.
Updated August, 31, 2018: The fix is live.