The real answer to my question is at https://github.com/xamarin/xamarin-macios/issues/12778:
If you updated to Xcode 13, you have 3 options:
You can download Xcode 12.5 from the downloads page in the Apple Developer Portal and continue to use it.
If you need to keep Xcode 13 but want to use our current stable d16-10 bits you will need to install the following in you macOS machine
Use a build from our xcode13-ios branch, this is where our next stable release is coming out and it includes all the iOS 15 SDKs APIs, that said we are still ironing it out.
The issue comments has feedback on what currently works and does not.