This is good to know. I don't care at all about using the latest iOS; if my iPhone 6S was still running the original version it shipped with, I'd never know the difference.
You probably would notice, because system apps like Safari are tied into it. I'm not sure how much of the web is broken on Safari 9. Or whether iMessage or Facetime still works.
I would say that's the one place where Android got it right, you can keep having updated system apps for a long time after the OS itself is abandoned (which is shamefuly quickly).
But I do care about security updates.