The primary hard part is testing the old versions. Xcode has decent backdeploy support (Xcode 26 supports targeting iOS 15, the final version to run on the 6S), but there's no way to actually verify that it works other than on an older device that's never been upgraded. It's a pretty substantial increase in testing burden and greatly increases the size of the pile of phones that you need to janitor for your CI setup.

Submitting apps to the app store requires using the latest version of Xcode (with a ~half year lag after a new one comes out), so it's now impossible to submit an update to the app store that supports iOS <15.