This app shows the TargetAPI of all your installed apps. This lets you check, which of your apps already support the latest features of your Android System.

If you are running Android 6 Marshmallow, this app helps you to check which of your apps already support the new granular permission settings.

If you are curious about the new cloud backup function:
On devices running Android 6.0 (API level 23) or higher, the default system behavior is to back up almost all data that an app creates. So any app should use the new backup mechanism. (If the developer did not choose to opt out of that feature.)

Guidelines from Google:
As new versions of Android are released, some style and behaviors may change. To allow an app to take advantage of these changes and ensure that the app fits the style of each user's device, an app developer should set the targetSdkVersion value to match the latest Android version available.