I suspect that this is something Dropsource may need to address so posting it here…
This email was just received, ironically as we are in the process of integrating OneSignal into our Dropsource iOS app (currently waiting on a push certificate)
Due to a change that may occur as part of the upcoming iOS 13 release, you must update to the latest version of the iOS SDK before building your app with Xcode 11. All of OneSignal’s wrapper SDKs including React Native, Unity, and Flutter have been updated as well.
The reason for this is that Xcode 11, which is being released alongside iOS 13, breaks a common technique that apps and libraries like OneSignal were using to get a push token for the device. If you do not use our new SDK then new users will not be able to subscribe to notifications from your app.
This unannounced Xcode change was identified by the OneSignal team as part of our process of thoroughly testing all beta releases of iOS prior to their public launch. As soon as we became aware, we filed a bug report with Apple and updated our SDKs to ensure guarantee ongoing compatibility. By updating the SDK before your next app release, there will be no impact to your app or subscribers.
Thank you for using OneSignal and our team is available if you have any questions about this.