App slicing is a good idea on paper. It allows users to save resources on their shiny toys by splitting the Apple data between the phone and the cloud.
Of course it requires some clever programming to make it work and Apple is having a lot of problems with the iOS9 operating system.
According to Apple its App slicing is currently unavailable for iOS 9 apps due to an issue affecting iCloud backups created from iOS 9 where some apps from the App Store would only restore to the same model of iOS device.
When a customer downloads your iOS 9 app, they will get the Universal version of the app, rather than the variant specific for their device type. TestFlight will continue to deliver variants for your internal testers.
Apple assures us that App slicing will be enabled with a future software update, although it is possible that having pulled it, we will never see its like again.