Apple announces that you will need to build your projects using the iOS 12.1 SDK when submitting new apps to the App Store.
Requirements for App Store
To take advantage of the new features provided by the iOS SDK, Apple will require that all new apps submitted to the App Store must be built with the iOS 12.1 SDK or later. This requirement takes effect starting March 2019.
This means that to submit any new apps, you will have to use Xcode 10.1, which provides the tools to build apps for the latest SDKs, including iOS 12, tvOS 12 and Swift 4.2.