People have started receiving this warning on App submissions today
ITMS-90809: Deprecated API Usage - Apple will stop accepting submissions of apps that use UIWebView APIs . See https://developer.apple.com/documentation/uikit/uiwebview for more information.
Would be good to have more information about it, like an official deadline.
Also the system used to warn is giving false positives, looks like it just searches for UIWebView word and doesn't really check if the app is using an UIWebView or not.