Stand by mode (always-on display)

As mentioned, Always On only works on iPhone 14 series and 15 series. This is a feature that was only intended for models released for those models and likely future models. It was never made to work retroactively on older models.

Yes it was absolutely a feature reserved for newer phones only and was never going to be available retroactively for older phones released prior to the iPhone 14 series. The reasons probably range with one obviously being to distinguish the new release and its feature set from previous models and of course be one of the entices to upgrade. Hardware may also play a part as well as how well everything would continue to work on an older device with lesser hardware (screen perhaps, chip used, etc). Much like newer ios updates on older devices (especially if it’s one more than 1-2 cycles behind) tend to show some slight signs of struggle (complaints of more lagging or faster battery drain seems to be the normal complaints) it’s possible even if some of these newer features were applied to older devices it might show more prominent struggles. So even if it were possible for those devices to use those features it might not be in the best interest performance wise. Ultimately only Apple really knows the reason why they go the routes that they do….anything we see publicly on the matter is pretty much going to be speculation or rumor no matter how solid the reference may seem.

Not sure if I buy the refresh rate reason. At once every second (1hz), the standby screen would not be able to display a smooth-motion second hand (as it does easily on my iPhone 12). Do we have a solid reference for this?

Frankly, I think this is simply a feature that has been placed on newer phones only.

If I’m wrong, please elaborate. :)

I think that always on is only available on iphone 14 & 15. My iphone 13 doesn't have that option either.

According to this, it has to do with the display on the 14 & 15 phones: The Super Retina XDR display on iPhone 14 Pro and iPhone 15 Pro devices can accommodate a refresh rate as low as 1 Hz, making them apt for an 'Always On' feature.