iOS and iPadOS 26 will run on most things that support iOS and iPadOS 18
Every year, Apple releases new versions of iOS and iPadOS, and most years those updates also end support for a handful of devices that are too old or too slow or otherwise incapable of running the new software.
Though this year's macOS 26 Tahoe release was unkind to Intel Macs, the iOS 26 and iPadOS 26 releases are more generous, dropping support for just two iPhone models and a single iPad. The iOS 26 update won't run on 2018's iPhone XR or XS, and iPadOS 26 won't run on 2019's 7th-generation iPad. Any other device that can currently run iOS or iPadOS 18 will be able to upgrade to the new versions and pick up the new Liquid Glass look, among other features.
Apple never provides explicit reasoning for why it drops the devices it drops, though they can usually be explained by some combination of age and technical capability. The 7th-gen iPad, for example, was still using a 2017-vintage Apple A10X chip despite being introduced a number of years later.