With the introduction of .NET Standard 2.0 support in Xamarin.Android, Xamarin.iOS and Xamarin.Forms the Ultimate UI for Xamarin 2018 Volume 1 product has been updated to .NET Standard 2.0. You can now reference this product in your .NET Standard 2.0 projects. You can no longer reference this product in Portable Class Libraries (PCLs) as these are being phased out in favor of .NET Standard.
Xamarin has a helpful topic about .NET Standard support in their product here. There’s also a blog on the Xamarin Help community site describing how you can update existing PCLs to .NET Standard projects. We recommend creating a brand new .NET Standard project from the Visual Studio 2017 project templates and then moving all of your code over if you are not familiar with manually editing .csproj files.
You can find more information regarding .NET Standard and support in Xamarin at the following links: