It's incredible the lengths Apple is willing to go in order to get people to update (can you even believe they just want that to brag about the number?) one: Nags cannot be disabled within iOS (TvOS Beta doesn't count. It isn't integrated into iOS.)I'm done pushing for the ability to downgrade to a previous version of iOS. I'll settle for an option to tell iOS "no longer notify me that an iOS update is available". As a workaround, I've installed the tvOS beta profile on my iPhone SE and iPad Mini 4 (too late for my 12.9 iPad Pro). Should Apple prevent that workaround from working, then that is the time I'm "done" with iOS devices.
Two: Nags are frequent if you charge the device with Wi-Fi on. Frequent as in multiple times a day. If I said no, chances are, 30 minutes later, the answer is still no.
Three: In every single support conversation, the first thing they always do is ask for the iOS version and try to get you to update.
This has a very clear example, although this was on Mac: https://forums.macrumors.com/threads/please-help-most-of-my-storage-disappeared.2089545/
Here, Apple wanted me to (listen to this) create a backup, update to High Sierra or Sierra or whatever it was at that time, delete my entire account and migrate everything to a new one.
The solution was... to turn Time Machine on and off.