I just got my v1 app fully converted over to WKWebView and my app got rejected like 7 times until I finally rid my app of or upgraded necessary plugins to get rid of any references to UIWebVew.
This thread helped a bit https://ionicframework.com/blog/understanding-itms-90809-uiwebview-api-deprecation/ - however I discovered the twitter-connect-plugin, that is not on that list, also had references to UIWebView. It needs to be added to the list.
The good news is that someone forked the project and created a new cordova-plugin-twitter-wkwebview which allowed me to keep twitter in my app.
My project is Ionic v1 still on cordova-ios@5.1.1 - so it is possible to get your app approved with those packages.