I have an iPad 3 with iOS 9.3.5. Now AFAIK, iOS 10 will not be releases for my device. After iOS 10 is released for other devices, soon Apple will stop signing iOS 9.3.5 firmware. So if after that, suppose I want to reset my device, i.e, restore it using iTunes, is that possible at all?
Apple will always sign "the latest available iOS for your device".
If they didn't, no-one with an iPhone 4 or older would ever be able to do a restore.