Regarding many of the other forum members' comments about why this isn't also coming to older iPhones and iPads, I am inclined to agree that it is an unnecessary move on Apple's part to seemingly software-lock this ability in what appears to be a rather petty move of forcing owners of older devices to upgrade to the latest and greatest model.
That being said, and just playing devil's advocate here, but is it possible this is actually not the usual Apple BS we think it is? I can't help but wonder if perhaps there is some legitimate reason they can't roll this out to older devices retroactively. Like maybe it has something to do with starting with a clean slate of this particular generation's battery assembly or something like that? We all know Apple wants control over as much of the supply chain and manufacturing process for all of their products and their individual components as possible, but could it be that they don't want to take the chance of getting it wrong with their battery partners so it was easier just to start with these latest generations where they can fully implement the health monitoring metrics exactly how they want to?
I'm certainly no chemical engineer or battery expert by any means but I know their inner workings and mastering quality control is nothing short of intricate, so that's why I'd be curious to hear from someone more knowledgeable than myself. Hell, even a short press release from Apple's PR department would be better than nothing.