You should make sure you don't have any more plugins that reference UIWebView, check for any old plugins for that. Make sure you use a recent version of cordova-ios too and cordova.
You could test this in your config.xml:
<preference name="WKWebViewOnly" value="true" />
You may as well try https://www.npmjs.com/package/cordova-plugin-wkwebview-engine-v2 because https://www.npmjs.com/package/cordova-plugin-wkwebview-engine is deprecated.
Edit: the official cordova blog recommended two years ago:
<platform name="ios">
<preference name="WKWebViewOnly" value="true" />
<feature name="CDVWKWebViewEngine">
<param name="ios-package" value="CDVWKWebViewEngine" />
</feature>
<preference name="CordovaWebViewEngine" value="CDVWKWebViewEngine" />
</platform>
and to use https://www.npmjs.com/package/cordova-plugin-wkwebview-engine