When Apple initially released iOS 13 and macOS Catalina, they had anticipated that they had provided developers enough time to comply with some requirements. These are:

• Apps must be built with the iOS 13 SDK, and use a Storyboard for the launch screen
• iPhone apps must support all iPhone screen sizes and iPad apps must support all iPad sizes
• Apple Watch apps need to be built with the watchOS 6 SDK
• Apps that support third-party sign-on services must also support Sign-In With Apple

Initially, Apple had wanted to have these in place by March 31st, 2020. However, due to Covid-19, many developers may not be able to accomplish this in the allotted time. Because of this, Apple has extended the deadline until June 30th, 2020.

If you are a developer, be sure to comply with these changes by June 30th, otherwise your app may get removed from the App Store.

Source: Apple Developer