Apple does not force iOS upgrades as the device owner has to accept the update manually therefore will continue working with iOS 9.
Actually Apple does enforce the iOS updates, like I said, I kept deferring updates for over a year leaving the original release of iOS that was present when the first iPhone 6es came out--because it's a bloody phone and that's all I use it for. Then one day when I *restarted* the phone without warning or accepting anything, the latest version installed itself without my consent.
Phones are a bit different from desktop or laptops though / which is why I don't run Windows 10. *cough*