I'm on iOS 15 and still regularly get updates for a handful of apps. How are they doing it?
This video was very informative on how Apple deliberately blocks developers’ ability to update an app without removing support for older iOS and an unsupported undocumented workaround the creator has noticed which at least momentarily works to work around it:
Your app must run on iOS 18 but you can drop features to get it to support as old of an iOS as you want.