I use a Visual Studio 2019 CE and a TFS 2018 Update 3.
I installed the Process Template Editor from the Marketplace: download site
Here you can edit the WITs like in the old editor. In the 'Workflow' tab I can edit and modify everything, I can even add new 'State' items. But It seems I cannot add a new 'Transition Link'. Has someone an idea how to make that work? Maybe I missed something or is this a bug?