Here's a heads up for an issue which I think has passed us by as blind IOS9 users until now. I've just discovered when you update to ios9 apps will no longer transfer from your device to iTunes and they will no longer be backed up onto your hard drive via iTunes either. The content will still back up but the app will not. This is a big deal because if you purchase an app via your IOS device and it is eventually updated to a less accessible version you will not be able to restore the earlier accessible version of the app via iTunes. This makes us much more vulnerable after paying out hard cash for accessible apps. Basically you will potentially be one update away from being screwed!
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App thinning
This is a side-effect of a feature called app thinning that was introduced in iOS 9, which saves space on iOS devices by only downloading the portions of the app that are applicable for the specific device. For example, an app with both iPhone and iPad interfaces will only need to include the iPhone interface when downloaded to an iPhone, since you will never see the iPad interface on that device. As the number of different devices and screen sizes increases, it makes sense for Apple to provide a way for developers to target multiple device types without the app becoming too large for users of each device.
Because an app may not be complete on your iOS device, iTunes cannot transfer the app from the device because it may not be able to install the app on another device you connect to it. Therefore, Apple chose to stop transferring apps from iOS devices to iTunes as of iOS 9. You can still download apps in iTunes from the iTunes Store and transfer the apps to your devices through iTunes, so if you're worried that an app you use frequently may have its accessibility broken, you can download the app in iTunes while a working version is available. However, this is very inconvenient if you have a large number of apps you wish to preserve, especially since you would have to repeat the process every time the app is updated.
I should also point out that some apps will eventually stop working if you do not update them to the latest version, especially if they access content online, such as streaming apps, so just because a particular version of an app is working today does not guarantee that you can stay on that version forever.