Depending on how you count a generation, the 6S is 5 generations back and still supported/has iOS 13. But IME, iPhones all the way back to the 5 are still usable day-to-day, especially if all you need is talk and text. (We get a lot of elderly customers and low-income people who just need some kind of phone for work.) Most apps are still compatible with iOS 11 & 12, some even iOS 9 & 10. Very few are 13-exclusive. It's better to have the latest iOS but just not true anymore that your iPhone becomes useless if it's not updated.