System.Xaml.XamlServices is not available on dotnet/runtime

Xaml Services is not available on .NET Core Runtime. It is only available on Desktop Runtime as part of the WPF.
Desktop Runtime is only supported on Windows OS, so the use of Xaml Services is available only on Windows.
Given that the base functionality of the Xaml services is not platform specific, it should be included in the base runtime and be available for all the supported OS of dotnet.
The concepts and capabilities of Xaml Services - reading/writing, type converters and markup extensions, etc - are so powerful that they can be used in an broader use cases than WPF.
One example for such use case is a library for developing Web applications. Using ASP.NET Core Runtime and Xaml for View definition, one can create multi platform web application that can run on both Windows and Linux.