Apple continues to lead the way with its excellent software support for older smartphones, unlike many Android OEMs. Ultimately, even Apple has a point where it drops iPhones from future iOS releases, and it seems like the iPhone 6s series will be on the chopping block as of iOS 15. The same will probably apply for the original iPhone SE, too.
https://www.notebookcheck.net/iOS-15-will-probably-not-be-headed-to-the-original-iPhone-SE-or-the-iPhone-6s-series.505943.0.html