Unfortunately, it's been confirmed today that there is no feasible way of mass deleting folders/projects as you can with tasks.
Tasks can be viewed in the List view & can be mass moved/tagged/assigned/copied/etc by selecting one, then clicking on "Select All" & selecting the command.
To do the same with Folders or Projects, I have been working from the left-hand blue area: typing a keyword at the top Filter area & selecting each Folder/Project ONE-BY-ONE, right clicking, & thereby deleting it. 😑
It would be great if a mass deletion technique can be developed for future deletion purposes...thanks!