I was able to use iMazing to backup my iPhone before updating. My simple milage app failed to work after the update to 14.5. Fortunately I have an iPhone 6S that is still on an older operating system. I restored ONLY the failed app to the 6S using iMazing and all is there working perfectly with the latest data. I was not able to update the app as it has not been in the app store for years.
Oh, interesting. So you were able to back up an app on one iPhone and then restore it to another even though it was no longer in the App Store. That’s good to know!