Existence Of Private Projects Is Visible
So, there's a way to see if someone has a private project registered under a name. Not a big deal, but still a bug. For example, if you open the "Run" URL of the testing project I made:
https://microstudio.io/TwiceUponATime/privateproject/
The response is "Project does not exist." But if you go a project I didn't make:
https://microstudio.io/TwiceUponATime/nonexistentproject/
Then the response is the standard 404 page. This works with any user and project, as far as I know.