Read the Docs supports 3 different privacy levels on 2 different objects; Public, Protected, Private on Projects and Versions.
Understanding the Privacy Levels
With a URL to view the actual documentation, even private docs are viewable. This is because our architecture doesn't do any logic on documentation display, to increase availability.
This is the easiest and most obvious. It is also the default. It means that everything is available to be seen by everyone.
Protected means that your object won't show up in Listing Pages, but Detail pages still work. For example, a Project that is Protected will not show on the homepage Recently Updated list, however, if you link directly to the project, you will get a 200 and the page will display.
Protected Versions are similar, they won't show up in your version listings, but will be available once linked to.
Private objects are available only to people who have permissions to see them. They will not display on any list view, and will 404 when you link them to others.