iOS 9 looks to free up space for OS upgrades
June 23rd 2015
Mike Beasley:
A new iOS 9 feature added in beta 1 was only discovered when users attempted to update to beta 2 earlier today. This new feature will allow the operating system to intelligently delete applications if you don't have enough free space to perform a software update. Once the update is complete, the apps will automatically be reinstalled and your data will remain intact.
This is a novel approach to a problem. I've got a 16 GB iPad for the kids and I always have to delete some apps to do even modest delta updates of iOS. Sounds like this feature will do what happens during a device restore -- basically re-downloaded from the App Store. Presumably the app data will either be backed up on iCloud or retained on the device.