I'm afraid it is difficulty..
You can download Xamarin.ios here : https://github.com/xamarin/xamarin-macios/releases/tag/monotouch-184.108.40.206
Then Check https://releases.xamarin.com/category/xamarin-ios/page/3/.
As this article mentioned , downgrading Xamarin.iOS to the version 11.8 in Visual Studio means that you need to downgrade VS for mac to the version 220.127.116.116.
However , you have to contact the support team for the download link .
Xamarin Inc., as a wholly-owned Microsoft subsidiary acting as a separate legal entity, adheres to the Microsoft Privacy Statement: Privacy & cookies