In .NET Core 3.1, DocumentDesigner class from System.Windows.Forms.Design.dll has all of its methods implemented as
new NotImplementedException(SR.NotImplementedByDesign);

The DocumentDesigner class is vital for custom hosting of Windows Forms designer in user applications. Judging from the class' .NET Framework implementation, it contains a lot of "glue" root designer logic, common for all document designers.

Will this class and other classes required for custom designer hosting be implemented in .NET Core?
What is the Microsoft's stated direction regarding the Windows Forms designer custom hosting features in .NET Core?

A variety of applications use this Windows Forms feature at the moment. They include report designers, application-specific data entry form designers, and others.