How to empty icloud drive recycle bin?

There is only a trash folder for a Mac , if you delete documents stored in iCloud Drive , the same will be removed from www.icloud.com and moved to the trash folder of Mac .

In case if you want to delete them , make sure you first archive them as they might be important for you .

See the last paragraph of this article Add your Desktop and Documents files to iCloud Drive - Apple Support

Then , select the documents by opening finder in iCloud Drive section , hold command key and tap the documents / folders and delete them .

Finally restart the Mac and empty the trash .

Try going to iCloud.com/Settings and in the lower left under Advanced, select Restore Files. Once that loads, put a checkmark next to the document you want to delete, and then click Delete.