Apple generally has a more nuanced and restrictive view of "what devices are capable", and are often adding new hardware to support such features, which many of the people who complain "they're withholding this on purpose!" simply gloss over. In this case, it appears that some existing iPhones can run the screen refresh rate down as low as 10 Hz, while the new screens may be able to run as low as 1 Hz (some other manufacturers have made phones with such screens).
I'm quite sure there will be people after the annual event(s) next fall loudly complaining "they're not supporting this in the old phones in order to force you to buy a new one!", when, in fact, the actual answer will be, no, they only added it to the new ones because the old ones would take ten times as much power to get the same effect, and Apple doesn't like putting in a bunch of work to implement what they think is a flawed solution. (I'm not saying everything they do is perfect - far from it - but they've shown a history of not wanting to start down a path they see as fundamentally flawed.)
Could they make a feature on the old phones that works sorta kinda like the one on the new phones? Yes, very likely so. And then people would complain about the battery life on the old phones dropping rapidly, "for no reason".