Please follow this thread: Received mail from Apple about the Deprecated API: UIWebView - #13 by boro-tech for updates regarding UIWebView.
A quick synopsis is that we are actively removing instances of UIWebView in our codebase and we hope to have a UIWebView-free version out as soon as we can without risking stability. So far it seems Apple is just warning users as several Expo developers have reported that their apps are still getting approved.
Cheers,
Adam