The post on What's New in the Revit 2020 API is overdue... for the time being, of course, you can refer to the documentation included with the Revit 2020 SDK, either Revit Platform API Changes and Additions.docx or the corresponding section in the help file
I promise you there is some really juicy stuff in there!
Here are some other quick notes as an Easter present for you
– RevitLookup for Revit 2020
– Revit 2020 SDK posted
– Generative Design with Philippe Starck...