In my case where each splash screen was individually designed instead of autogenerated or laid out in a story board format, I had to stick with my Legacy Launch screen configuration and add portrait and landscape images to target iPhoneX 1125×2436 orientations to the config.xml like so:
<splash height="2436" src="resources/ios/splash/Default-2436h.png" width="1125" />
<splash height="1125" src="resources/ios/splash/Default-Landscape-2436h.png" width="2436" />
After adding these to config.xml ("viewport-fit=cover" was already set in index.hml) my app built with Ionic Pro fills the entire screen on iPhoneX devices.