They can probably enable it for older iPhones and this is just choice, but then again imo, you can optimise any iPhone for battery life by yourself just fine.
And regardless, the #1 battery killer is still brightness (and iOS updates).
I’ve been getting Apple’s 16 hours of SOT - Apple’s claimed specs - on my iPhone Xʀ since day 1. It’s running iOS 12.
There are no secrets. Keep the original iOS version, disable heavier settings, lower brightness and use efficient apps and you’ll hit Apple spec.
This has been true since the original Plus models. (I’ve found that Apple’s claimed 10 hours of SOT for the iPhone 6s were practically impossible to achieve. 8.5 was consistently doable on iOS 9, though).
In spite of that... it’s still a little funny for me that many people STILL don’t know that high brightness is the #1 independent battery killer and that they STILL ask “what’s wrong with my battery? I’m using my brand new iPhone 16 Plus at full brightness with 5G and Maps, and playing some games on it. Why is my battery life so poor”? 19 iOS iterations and iPhones since 2007 and many people still can’t associate full brightness and/or power-intensive apps with poor battery life.
Point is, don’t worry folks, you can optimize this by yourselves.