You can still sync apps FROM iTunes TO an iDevice running iOS 9. That is what I've been doing. This keeps my iTunes app library up to date with iOS 9 compatible apps that can be synced to other iDevices running iOS 9 without downloading them multiple times.
The way this works is.... I check my iPhone (iOS9) for updates. If I see any, I launch iTunes and update the Apps in my iTunes library. Then I sync my iPhone via iTunes. This replaces the out of date apps on my iPhone. There are a few quirks. For example, the App Store never shows these apps in the list of those that have been updated. And my iPhone doesn't show the little blue dot. But the version numbers change and the apps open with "what's new, etc". What is not clear is whether the apps synced from iTunes are universal or "thinned".
The reason I'm going to this trouble is that it keeps the iTunes app library up to date for when I upgrade our other iToys to iOS 9. This cumbersome process really only makes sense if you have a slow and/or capped internet connection.
---GetRealBro