Hey,
the exact future of gRPC support on Xamarin is a bit unclear at this point. AFAIK, net60 (which will be released later this year) should be the unification point for Xamarin and .NET - so starting from net60 it should not matter nearly as much if you're targeting .NET Core or Xamarin. That also gives us the chance that Grpc.AspNetCore.Server will "magically" start working on Xamarin - we will find out later this year.
Ad Xamarin.Android on net50 - AFAIK that's not enough. Xamarin.Android on net60 should be different.
Ad Grpc.Core's support for Xamarin: We're not planning to remove the existing packages so you if need, you can continue using them (there just won't be new releases). From the perspective of official support, this actually doesn't change much because the Grpc.Core's support for Xamarin is (and always has been) documented as "experimental only" and we weren't providing any guarantees that it
works (even though in practice it works pretty well to my knowledge) - so even your current use of it a bit of a grey zone since there's no official support for Grpc.Core.Xamarin.