As a computer engineer, I'm genuinely confused as to why it didn't come to more recent devices.
This is a feature that is super easy from a software perspective. It is also not a feature that you can really market in a sense to entice people to upgrade, so there's no sense in gatekeeping it.
Same with the battery cycle information you can see in the 15s.
This is the type of feature that is a small nice little touch that is a no-brainer for apple to add to older iPhones. But for some reason, they haven't, and no one can seem to figure it out for sure.
Apple being Apple probably has a reason they think is good enough. Whether we as a consumer base would agree that's a good reason is a different story.
Best case scenario: it actually is a hardware change in addition to the firmware & software.
Worst case scenario: they have some dumb reason for not doing it.
Wild case scenario: Lightning isn't capable of doing it properly so it's limited to USB C iPhones.
Side note: What about iPads then?! Eh not as big of a deal IMO. iPads generally have pretty good battery life and aren't charged as frequently (because they're used less on average probably in most cases) so they don't really benefit from extra battery protections like this. Sure it would be nice if you had the option at least, and in an ideal world we would, but I'm not surprised it's not on iPads (or Macbooks for that matter).
C'est la vie. No reason to lose sleep over it quite frankly.