Just because Apple pushes iOS/macOS upgrades/updates to their devices and computer, doesn’t mean you are forced to install them.
All of my iOS devices that are capable of upgrading to the current iOS are all still on iOS 10.3.3.
iOS 10, like iOS 9, has been running and working stable across all my iDevices with nary an issue.
So, I choose/chose stability.
There was really NOTHING I really wanted in iOS 11 and so far only a handful of things I was looking forward to in iOS 12.
My Mac has only been upgraded and maintained on OS X 10.11.6 El Capitán, but I regularly run and use the older OS X 10.9.5 Mavericks.
YOU DO have a choice.
Sure, many apps I use are not current, but they all still work and I can still use these without issue.
For the time being, small price to pay for a device that is still working as well now as it was three years, ago!
My opinion that since Apple's big push to 64-bit hardware/software iOS ecosystem, both iOS 11 and, so far, iOS 12 have been garbage. iOS 11 WAS REALLY total garbage and while iOS 12 appears to be better in some respects, there are STILL glaring issues and bugs that have kept me from upgrading my iDevices to iOS 12 when I really wanted to be able to upgrade, this time around.
Mainly for group FaceTime and things like the new AR-based Measure app.
Apple has forced me, once again, and for the near future, to have to wait out another major iOS version to see how it is going to shake out in the end.
For some reasons, totally unknown to my own tendencies and sensibilities, everyone with Apple mobile products and computers always just stupidly anf blindly upgrade/update their Apple devices and computers immediately, if not sooner, and end up living with the results. Good or bad.
The days of iOS/macOS upgrades/updates just working, with minimal issues, is gone!
Since iOS 8, I have look at iOS device upgrades/updates with extreme caution and, usually, wait out most of the iOS version update cycle before making the major decision to upgrade my expensive Apple devices and computers.
Once again, you DO have the choice to upgrade/update iOS devices and since Apple makes this a one-way only move, Apple iDevice users should really think hard before performing ANY iOS upgrades/updates.
FYI,
For future reference there is a way to temporarily remove any iOS upgrade/update so it can't be downloaded at all or by accident.
Go into the Settings App, General Settings panel.
In the right hand column, tap Storage ( and iCloud Usage, if still using iOS 9 or 10 ).
At the top in the first block of 3 panels where it is titled at the top of this block Storage, tap the panel that says Manage Storage.
A list of every app will generate itself.
Once this list stops generating, look through this list until you see something like iOS 10/11.x.x in this list.
Tap on the gray disclosure arrow at the far right on that iOS 10/11.x.x panel.
It will bring you to another screen that will allow you to completely delete/remove the entire iOS upgrade/update.
This removal/deletion of an iOS upgrade/update is not permanent and may return at anytime when the iOS upgrade/update at some point will get re-pushed to your iDevice, where you can simply perform this procedure, again.