So you've uploaded your latest build based on the React Native framework and BEHOLD - you've received a mail from Apple that reads:
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.
First thing first
Please bear in mind that this is just a warning for now. The build was delivered and processed correctly. In some point in the future Apple might decide to start enforcing this deprecation and reject apps based on this.
Identifying the culprit
Considering that you've been a well behaved developer and stopped using UIWebView in your own code, the main issue here is identifying the offending library that is still using it. this might prove to be a pain in the ass since many different libraries might be using a webview behind the scenes.
grep is to the rescue. Run this line at your project's root directory to find out which exact dependency introduced UIWebView to your code:
grep -r UIWebView node_modules/*
And here is grep for Windows.
The culprit here at Hippotec seems to be the same one across multiple projects:
node_modules/react-native-device-info/ios/RNDeviceInfo/RNDeviceInfo.m: UIWebView* webView = [[UIWebView alloc] initWithFrame:CGRectZero];
Luckily, the react-native-device-info crew are working on a fix.