iOS 15 will probably not be headed to the original iPhone SE or the iPhone 6s series
Apple provides much longer software support for its smartphones than Android OEMs do, but even Apple has a cut off date for iPhones. The iPhone 6s and iPhone 6s Plus support iOS 14, for instance, despite having been released over five years ago. Both devices can be upgraded to iOS 14.2 too, so Apple is not just leaving them on iOS 14. The same applies to the original iPhone SE, which arrived in March 2016.
However, a new report by The Verifier claims that Apple will only distribute iOS 15 as far back as the iPhone 7 series, meaning that the iPhone 6s series and the iPhone SE will miss the cut. Nonetheless, that should give the iPhone 6s series six years of software support and the iPhone SE five and a half, which remains market-leading.
Correspondingly, iOS 15 should make its way to the seventh generation iPod Touch and newer devices like the iPhone 8 series and iPhone 8. We would expect newer versions of Apple's other mobile OSes, iPadOS and watchOS, to arrive in September next year alongside iOS 15. Presumably, Apple will drop support for the iPad Air 2 as of iPadOS 15 and the Apple Watch Series 3 with watchOS 8, but the company will probably not announce which devices will be eligible for either OS until WWDC 2021.