My iPhone 6 Plus came with iOS 8.0 factory installed. I kept up with the updates until 10 came out, at which point I've refused every update. So my phone sits at 9.35, likely forever.
Reason (aside from avoiding bugs like in this thread topic) is to avoid a bloated OS that eats your battery life and overtaxes the processor/gpu. On a previous Apple device, I noticed that as soon as you got to two OS revisions above what it came with originally, it gets painfully slow. On that particular device it was originally iOS 4, and when i updated it to 6 there was a dramatic slowdown in responsiveness and app loading time. I eventually updated it to 7 to see if that helped, but it did nothing aside from taking up more disk space. It became so unusable, I gave it away for free.
There is a problem with this strategy though. Apple will force you to update. No matter what you do to your phone setting, it will phone home and automatically download the latest update. Disabling "Settings: App Store Auto Update" has no affect on this whatsoever. There is no way to disable this, aside from:
1. Jailbreaking with Cyanogenmod.
2. Block Apple update server from your wifi router (your phone will only attempt to auto-download when on wifi and not when on cellular)
I went with option #2 since I did not want to deal with the hassle of jailbreaking. Simply go to your router config page and block these two URLs: