Developers should be submitting their new apps tested on the latest versions of the iOS and WatchOS operating systems now, says Apple. Additionally, it has set a date for when all new apps must be built using the most recent SDK releases.
Apple will release the final, public version of iOS 12 and WatchOS 5 on September 17, and writes that developers working on apps with features particular to those versions should submit them for approval now.
Furthermore, developers should be aware that from March 2019, all new apps submitted should be built with the iOS 12 SDK, and support the new, larger iPhone Xs Max. The new version has a 6.5-inch screen, making it the largest iPhone Apple has made.
The Apple Watch Series 4 will make use of WatchOS 5 in new ways
It’s the same situation for WatchOS. Also in March 2019, Apple requires new Watch apps to be built with the WatchOS 5 SDK and to work on the new Apple Watch Series 4, which also has a larger screen than the previous Watch models.