If old projects aren't being actively used, but their data is still important, you could archive them.
Archiving a project is simply done using the actions (...) available on projects page
Once archived, the project is not shown on the projects page or on other pages that list up open projects, but can be accessed using the "Archived Projects" link in navigation.
Reopening a project
Archived projects can be reopened to make them usable again.