Android has long allowed designers to draw on top of various other apps and the system URINARY INCONTINENCE. This is how Twilight adjusts the color of the display, and additional apps that overlay matters on your status bar. A big change to the way Android U handles overlays could split features of these apps, object rendering some of them essentially useless.
As explained by developer James Fenn, whose app Status is affected, Google is deprecating TYPE_SYSTEM_OVERLAY for developers.
Android O feature spotlight: Apps can no longer draw along with system UI was written by the awesome team on Android Police.