Android O breaks apps like Twilight and Status that draw over the system UI
You will be greeted by this notification upon installing Twilight on Android O
In the case of Twilight, this means that the app won't be able to apply its warming filter over the status bar, nav bar, lock screen, and notification shade, among other parts of Android's interface. Status, on the other hand, becomes completely useless, as the app draws exclusively over the status bar.
A Google rep has already confirmed that this is in fact Android O's intended behavior and not something caused by a bug. If no way around this is found, Status developer James Fenn says he may be forced to discontinue support for his app on newer versions of Android.
This is not something that's going away for the final release of Android O and has likely been done with security improvements in mind.
sources: Google (1) (2); James Fenn via AndroiPolice
Things that are NOT allowed: