I have Sketchup Pro 2020. I am trying to download models from the 3D Warehouse that are compatible with older versions. I understand that 2017 will no longer be available, but I am not finding any options to download anything other than 21. Usually I hit the download button and I am given the option to choose an older version; but now it automatically downloads the 21 version. Is the option to download older versions of the models no longer available?
What browser are you using? Which model exactly? Could you provide a link to?
Normally, the “versions selection” menu should appear:
( 
Or are you trying with the built-in 3DWH browser inside Sketchup ? )