The Xamarin.iOS Troubleshooting documentation suggests it is possible to set which version of the Xamarin.iOS SDK to use, but when following the paths listed there is only an option for setting the Xcode path. https://docs.microsoft.com/en-us/xamarin/ios/troubleshooting/questions/ios-sdk
For context, I am attempting to set up a Mac to support targeting either iOS 12.4 or 13.1. While I have figured out how to enable selecting an SDK version in the iOS Build tab in Visual Studio, the build fails because it is using the Xamarin.iOS 13 SDK regardless of the native iOS SDK being used.
(_CompileToNative target) ->
/Library/Frameworks/Xamarin.iOS.framework/Versions/13.2.0.42/src/Xamarin.iOS/CLLocationManager.g.cs(1053): error MT4162: The type 'CoreLocation.CLBeaconIdentityConstraint' (used as a parameter in CoreLocation.CLLocationManager/_CLLocationManagerDelegate.DidFailRangingBeacons) is not available in iOS 12.4 (it was introduced in iOS 13.0). Please build with a newer iOS SDK (usually done by using the most recent version of Xcode).
/Library/Frameworks/Xamarin.iOS.framework/Versions/13.2.0.42/src/Xamarin.iOS/CLLocationManager.g.cs(1077): error MT4162: The type 'CoreLocation.CLBeaconIdentityConstraint' (used as a parameter in CoreLocation.CLLocationManager/_CLLocationManagerDelegate.DidRangeBeaconsSatisfyingConstraint) is not available in iOS 12.4 (it was introduced in iOS 13.0). Please build with a newer iOS SDK (usually done by using the most recent version of Xcode).