The Windows Forms Out-Of-Process Designer (or the OOP designer) is currently available as a pre-release Visual Studio feature in Visual Studio 2019 or later. You can enable the OOP designer via Tools -> Options -> Environment -> Preview Features with Use the preview Windows Forms designer for .NET Core apps. Releases of .NET Core Windows Forms Designer are tied to Visual Studio Preview releases, so update to the latest Visual Studio Preview version to get the latest designer features.
For the latest news on the designer please refer to the series of blogs discussing the current state of the designer.