System.Windows.Markup.XmlnsDefinitionAttribute in .NET Framework 4.7.2 before and I cannot find the corresponding type in .NET 6.0.
MSDN – XmlnsDefinitionAttribute says that it is not supported in .NET 6.0.
How can I map a XAML namespace to a clr-namespace in .NET 6.0?
WPF was part of the monolithic .NET Framework. Since .NET Core it was split up into separate runtimes, see downloads for an overview. Since then there is a core runtime that does not include WPF and a Destop Runtime that does (there is a separate ASP.NET Runtime as well). In the documentation, when you read Windows Desktop, the .NET Desktop Runtime is meant, so yes, the attribute is included in .NET 6 according to the documentation.