Downgrading Xamarin components is hard. Let's make it easier.
This is a by-no-means-complete collection of direct links to Xamarin components. These links come with no guarantee!
Happy downgrading.
https://dl.xamarin.com/VsMac/VisualStudioForMac-8.8.4.30.dmg
https://dl.xamarin.com/VsMac/VisualStudioForMac-8.8.3.16.dmg
https://dl.xamarin.com/VsMac/VisualStudioForMac-8.7.8.4.dmg
https://dl.xamarin.com/VsMac/VisualStudioForMac-8.5.3.16.dmg
https://dl.xamarin.com/VsMac/VisualStudioForMac-8.5.2.13.dmg
https://dl.xamarin.com/VsMac/VisualStudioForMac-8.5.0.3183.dmg
https://dl.xamarin.com/VsMac/VisualStudioForMac-8.3.7.1.dmg
https://dl.xamarin.com/VsMac/VisualStudioForMac-8.3.4.8.dmg
https://dl.xamarin.com/VsMac/VisualStudioForMac-8.3.1.18.dmg
https://dl.xamarin.com/VsMac/VisualStudioForMac-8.0.9.5.dmg
https://dl.xamarin.com/MonoTouch/Mac/xamarin.ios-14.6.0.15.pkg
https://dl.xamarin.com/MonoTouch/Mac/xamarin.ios-14.0.0.0.pkg
https://dl.xamarin.com/MonoTouch/Mac/xamarin.ios-13.16.0.13.pkg
https://dl.xamarin.com/MonoTouch/Mac/xamarin.ios-13.14.1.39.pkg
https://dl.xamarin.com/MonoTouch/Mac/xamarin.ios-12.16.1.24.pkg
https://dl.xamarin.com/MonoTouch/Mac/xamarin.ios-12.14.0.114.pkg
https://dl.xamarin.com/MonoforAndroid/Mac/xamarin.android-11.1.0.17.pkg
https://dl.xamarin.com/MonoforAndroid/Mac/xamarin.android-11.0.2.0.pkg
https://dl.xamarin.com/MonoforAndroid/Mac/xamarin.android-10.2.0.100.pkg
https://dl.xamarin.com/MonoforAndroid/Mac/xamarin.android-10.0.6.2.pkg
https://dl.xamarin.com/MonoforAndroid/Mac/xamarin.android-9.4.1.0.pkg
https://download.developer.apple.com/Developer_Tools/Xcode_12.3/Xcode_12.3.xip
https://download.developer.apple.com/Developer_Tools/Xcode_12/Xcode_12.xip
https://download.developer.apple.com/Developer_Tools/Xcode_11.6/Xcode_11.6.xip
https://download.developer.apple.com/Developer_Tools/Xcode_10.3/Xcode_10.3.xip