Should we add [assembly:Introduced (...)] to the platform assemblies ?

[spouliot]

Right now our minimum supported OS are not defined inside the assembly metadata (but inside our tools / build scrips). However we'll need this for dotnet so it make sense to consider doing the same for legacy assemblies.

Beside the slight change to the attribute itself

diff --git a/src/ObjCRuntime/PlatformAvailability2.cs b/src/ObjCRuntime/PlatformAvailability2.cs
index 04477a67d..c5c5390a9 100644
--- a/src/ObjCRuntime/PlatformAvailability2.cs
+++ b/src/ObjCRuntime/PlatformAvailability2.cs
@@ -51,6 +51,7 @@ namespace ObjCRuntime
        }
 
        [AttributeUsage (
+               AttributeTargets.Assembly |
                AttributeTargets.Class |
                AttributeTargets.Constructor |
                AttributeTargets.Delegate |

We would need to mark API that are not available differently.

E.g. All ARKit.* types would need to mention they are not available outside iOS, otherwise the current (legacy) tooling would presume it's available in the minimum version (since the lack of attribute on the type would fallback to the assembly, which would mention something with [Watch (2,0)]).

This has indirect benefits since we often inline (for protocols) several [Introduced] attributes in members - but if we had an [Unavailable] on the type then we could (actually would) skip them.