In general when something is no longer needed I’d recommend to rather archive instead of deleting.
Now for projects that have been deleted by mistake usually an email is sent so it can be restored, otherwise reach out to support@asana.com and they should be able to help.
To prevent users from being able to delete a project here is an existing feedback request thread:
Archived projects could then be moved into a separate team where you store all archived projects.
Archiving teams is not possible yet, here is a feedback request thread in case you are interested.