Security updates are irrelevant imo, the issue is that if you want a good experience you won’t get one on iOS 15.
Battery life is utter garbage, and performance would not be up to standard for me, I don’t tolerate garbage.
I don’t have one (although I do have a 6s on iOS 10), but excluding the camera quality, I’d be content with using an iPhone SE 1st-gen on iOS 10. It wouldn’t be usable for compatibility reasons because developers are trash at supporting older iOS versions, but if compatibility were magically maintained forever I would happily use one.
The camera is the only aspect which would bother me, I use an iPhone Xʀ on iOS 12 and it’s time to upgrade for two reasons: compatibility, and the camera. It lacks too many features, and I even resented that back when the 11 was launched, it was so much better!
But apart from that, I’d use one if it were compatible, possibly my favourite iPhone ever launched (maybe behind the iPhone 5 on iOS 6).