I'm afraid it is difficulty..
You can download Xamarin.ios here : https://github.com/xamarin/xamarin-macios/releases/tag/monotouch-18.104.22.168
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 22.214.171.1246.
However , you have to contact the support team for the download link .
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