Apple has always implemented a pretty strict policy regarding applications that may appear in the App Store. One of the new rules that will soon be implemented is that all applications must be built using iOS 13 from April 2020.
This actually makes sense considering that all applications in the App Store should be able to adjust to the new iOS version released by Apple. Application developers naturally need time to update their applications on the App Store.
Meanwhile, iOS 13 was introduced by Apple in June 2019 and released a Beta version to registered developers since WWDC 2019 ago. Meanwhile the final version of iOS 13 will be released by Apple on September 19, 2019.
Developers should have adjusted their iOS application from the start when the iOS 13 Beta version was released by Apple. Thus, they have enough time to make incremental changes and are ready to be released to the public when iOS 13 is released.
For you developers who already have an iOS app on the App Store, like it or not you have to immediately update your app on the App Store before April 2020.
For developers who are just about to release an application on the App Store or even just about to make an iOS application, from now on you have to build the application with the iOS 13 SDK.