Additionaly, WKURLSchemeHandler support has been introduced with this release. Using a custom scheme to serve your app content through fixes CORS issues that exist because of the strict security policies that WKWebView has applied to the file scheme. You can easily configure your Cordova project to use a custom scheme by setting the preference options scheme and hostname in the config.xml file.
I just submitted an update last week with 5.1.1 with this plugin built on VoltBuilder and it was approved. Apple only requires WKWebView that I know of? Cordova 6+ causes some serious issues that will take awhile to sort out for me. What is the reason that you are not allowing this plugin now?