cannot find protocol declaration for NSFileProviderItemDecorating

This problem has been fixed by today's Visual Studio for Mac update which adds support for ios14.

Check for updates... Install updates and all worked fine again :-)

...you need to use a new Xamarin.iOS to support a given Xcode, as 13.20 is not that.

https://developercommunity.visualstudio.com/content/problem/1097251/cannot-find-protocol-declaration-for-nsfileprovide.html

I believe the same to be true of 13.99 and Xcode 12 right now per OP's statement.

Even though Xamarin.iOS 13.xx is the latest stable it appears it does not support Xcode 12 yet.

Downgrade Instructions

You need to downgrade XCode. I tested on 11.5 and could build but I think anything 11.7 or lower should be fine. Delete your Xcode, restart and then download an older version from here https://developer.apple.com/download/more/ to revert. You need a developer account to access Xcode archives.

I don't think you need to revert Xamarin.iOS as long as you are on 13.99< but if you do you can find the packages here https://github.com/xamarin/xamarin-macios#downloads they will just overwrite your current packages without any uninstalls just make sure to close visual studio before running the package.

My Working Build Spec

Apple Developer Tools Xcode 11.5 (16139)

Xamarin.Mac Version: 6.16.0.13 (Visual Studio Community)

Xamarin.iOS Version: 13.16.0.13 (Visual Studio Community)

Can confirm that after the new update to Visual Studio 2019 version 16.7.4 (Windows) I was able to get proper errors, and eventually build the solution as expected.

Thanks Alan Beech above for this original working answer!