Some good news. The issue that had been introduced by changes to comply with more recent versions of Chromium, has been fixed. SketchUp 2017 and 2018 are showing the 3D Warehouse window contents ok now.
For 2020 Design in particular, I found that I needed to go to https://3dwarehouse.sketchup.com/ in the Chrome browser to do the sign in step, then the window in 2020 Design worked ok. You may need to even sign out, then sign in with Chrome.