Closed Projects CSV Export
Closed Projects can be exported then deleted, to clear the system.
Goto the web app, Closed section from the Left Pane menu.
Select some or all projects, then click on CSV Export.
The CSV includes the Project details, as well as Milestones, Tasks and Notes; but not Files or Froms, or other linked items.
After downloading the CSV, you can also delete Closed projects from the same screen.