Shortly after the introduction of iOS 13, Apple decided to send out warning messages about the UIWebView deprecation. The warning would read:  
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. After you’ve corrected the issues, you can use Xcode or Application Loader to upload a new binary to App Store Connect.
So, basically, starting from April, you need to strip your project from referencing the old UIWebView. 
You have done good to update to 4.5.0.617, but this is not enough for the fix. You need 2 more things:
- To have Linking set to 
SDK Only or All 
- To add an additional mtouch argument 
--optimize=experimental-xforms-product-type 
A detailed explanation from Microsoft can be found here.
You can read more about the storage.ide file here: What is the "storage.ide" file beneath my Visual Studio solution folder, and what is "persistent storage"?
Long story short - everything in the .vs folder should be excluded from source control and it won't affect the WebView warning.