You can now submit apps that take advantage of the powerful new capabilities in the next release of iOS, watchOS, and tvOS. Build your apps using Xcode 10 GM seed, test with the latest releases of iOS 12, watchOS 5, and tvOS 12, and submit them for review.
According to Apple, all new apps and app updates for iPhone, including universal apps, will be need to be built with the iOS 12 SDK and will need to include support for the iPhone XS Max starting in March 2019.
All new apps and app updates for Apple Watch will need to be built with the watchOS 5 SDK and will need to support Apple Watch Series 4 by the same date.
Apple plans to release iOS 12, watchOS 5, tvOS 12, and new software for the HomePod on Monday, September 17. Apple will release macOS Mojave a week later on Monday, September 24.