As developers (and early-bird beta testers) have been working with the various preview releases of iOS 12 this summer, one of the big changes they've seen are performance improvements, particularly for older and/or lower-powered devices.
This bodes well for this fall's updates, given that Apple's track record isn't great when it comes to how older hardware handles new iOS releases.
After ensuring that all recent – and some not-so-recent – hardware can run the latest iOS version, Apple has moved a number of devices off the update train.
[ Related: How to get Apple's iOS 12 or macOS 'Mojave' betas ]
The same holds true for macOS Mojave, which is the first macOS release in a while to axe support for some older Macs.
(Recent macOS upgrades have tended to keep the same hardware/device requirements as their immediate predecessors.)