Since I’ve submitted the ticket to Mendix Support, here is the answer:
“Regarding the warning, ITMS-90809: Deprecated API Usage - Apple will stop accepting submissions of apps that use UIWebView APIs : We checked with our R&D department and the native apps do not rely on the UIWebView. Our R&D department is working on a fix to get rid of this warning, but it should not block your app at this point.
The warning can safely be ignored, for now. You will be able to get your application approved for the App Store.”
Also, I can confirm that Apple approved the applications I submitted for review a few days ago.
Apple did an announcement on this: https://developer.apple.com/news/?id=12232019b
New apps are no longer accepted from April 2020 and app updates are no longer accepted from December 2020.
Good news, Cordova seems to have fixed this: https://cordova.apache.org/announcements/2019/11/25/cordova-ios-release-5.1.0.html We will also work on making sure you can keep publishing your hybrid apps.
Additional info from Mendix support regarding this subject:
“We checked with our R&D department, and it turns out that the PhoneGap/Cordova core library is referencing the old UIWebView. This is not something Mendix can fix, this has to be fixed by PhoneGap/Cordova. For native apps, Mendix can fix this, as we don't rely on PhoneGap/Cordova for native apps.”
This one has been solved with the “Hybrid App Base 5.0.0”