Are you basing this on real experience or just fishing for HN anti-Apple upvotes?
Just asking because my kid is using an iPhone 6s with iOS15 and really experiencing absolutely no problems whatsoever.
I'm in the same boat, my kid is now the third owner of a handed down 6S and it still does everything without being slow or lacking app support. The features it is missing are mostly neural engine related and are conveniences not vital functionality. I could 100% use that phone as a daily driver and not be frustrated if I swapped in a new battery.
Anyway I have used a 6s on recent versions of iOS. It works but don’t kid yourself if you think the experience in any way compares to using a recent device.
I have had an oddity in the last year or so (so iOS 14 and onwards?) is that music playback software seems to get background killed (OOM?) fairly often if it isn't playing. Main issue for me is my general pattern with driving would be to cue up music / podcasts beforehand, pause it and when I select bluetooth for sound in car it would start playback. Now I seem to have to be careful not to actually do anything with my phone (like check websites or whatever) after preparing audio, otherwise there's a strong chance you'll just get nothing.
So not perfect, but this may be a weird edge case I'm running up against. Doubt the phone is going to get iOS 16 though, so will have to think about what to do.
I think those first few years of iPhone where phone power was really leaping up between models, and iOS was jumping in complexity alongside it did give Apple bit of a bad reputation. We got software updates (good) but they just didn't perform at all well on older phones (bad). It seems to have settled down a bit nowadays where iOS isn't outgrowing the old hardware as much. But of course then we had the infamous "people think we're intentionally slowing down old phones, lets ship a feature that protects the battery by intentionally slowing down old phones and not tell anyone" thing.