Version 4 (V4) of the AWS SDK for .NET has been released!
To start using the new version of the SDK, see the AWS SDK for .NET (V4) Developer Guide, especially the topic for Migrating to version 4.
Xamarin projects (new and existing) must target .NET Standard 2.0. See .NET Standard 2.0
Support in Xamarin.Forms
Also see the information about Portable Class Library and Xamarin.