Developer David Barnard tweeted today that Apple is requiring that he remove direct links from his app Launch Center Pro because the app uses "undocumented APIs" to launch built-in Apple apps.
According to Barnard's tweets:
Apple called to let me know that we are using “undocumented APIs” in @launchcenterpro and that we need to remove certain Apple app URLs. 
The URLs will still work, but we have to remove Calendar, FaceTime, FaceTime Audio, GameCenter, Music, Passbook, Videos, Reminders, and iTMS. 
To be clear, these URLs will work just fine unless Apple actually removes them from their respective apps. 
All of those URLs have one thing in common, they are apps that ship with iOS. Apple apps in the App Store seem to still be fair game. 
And to be fair, those URLs *are* technically undocumented. I’m not even sure how they were discovered by people who shared them with us. 
Launch Center Pro is a speed dial-esque alternative launcher that uses hidden URLs to quickly open apps. Because the URLs to Apple's built-in apps are undocumented (with the exception of FaceTime), app developers are forbidden from using them on the App Store.