I got this on a reply from a service request when I asked the same thing just the other day.
“Unfortunately, game projects cannot be deleted at this time due to limitations in our UDN integration, but this should be available in the new future as well.”
I found a way to do it, I created a new organization with a random name.
Then I transfered all the projects I wanted to delete to that new organization and archive them all.
After that I just deleted that new organization with all the projects inside and that’s it.
I could delete one of my projects If I to documents in the file explorer, you may be able to see the name of your project on a unzipped file. If you delete that, it is also deleted from unity, But if you saved the file somewhere else try deleting it there.
Hope this was helpful!