How do I backup and restore my IPA app backups in iTunes 12.7?
This looks reasonably promising - but I don't know how it will work for future releases...


  Manually add items from your computer
  
  If you have media on your computer that you can't redownload from the
  App Store or iTunes Store directly on your iOS device, such as
  custom ringtones or iOS apps, you can drag and drop the media from
  your computer to your iOS device using iTunes.
  
  
  Connect your device to your computer.
  Open iTunes. Make sure that you have the latest version.
  On your computer, find the item you want to add to your device. If you're looking for Apps, Tones, or Books (for Windows) that previously
  appeared in your iTunes library, you can find them in your iTunes
  media folder.
  Drag the item from your computer to your iOS device in the left sidebar of iTunes.