That includes Super Retina resolution, and interface accommodations for the notch, Apple said in an email to developers on Thursday. When publishing through iTunes Connect, apps will require iPhone X screenshots, even if they're just produced through Xcode's iPhone X simulator.
Apple has yet to set an equivalent deadline for app updates.
Today's notice is likely a signal that that Apple considers the iPhone X the new template for the iPhone moving forward. The company is expected to ship three new iPhones this fall, including 5.8- and 6.5-inch OLED models and a 6.1-inch LCD variant. All of these may include some sort of display notch, since they're expected to offer Face ID across the board.