Show all Visibility Level options in a group
Proposal
If a group is public and you'd like to then make it private for any reason, all projects and sub-groups within that group must also be private. If there is a single project that is still public, you won't get to see the option "Private" under Settings > General > Visibility level. In fact, you'll only see "Public" with no option to change it. This can be confusing if you're not clear how the visibility level works across projects and groups.
Considering the interest of this feature it seems that it may be useful to be explicit in the user interface since it is not possible to make a single project public within a private group.
Visibility Level settings when you are unable to change it to private
Visibility Level settings when it is possible to make it private
It could possibly make it more clear to the user that they are unable to make a group private by still showing the option and graying it out. Perhaps there could be a tooltip or note that says "You still have projects or sub-groups that are public. Make them private in order to make this group private." or something along these lines.
Docs for reference: