Once that option is checked, the system certainly works to completely hide sub-forums on all the forums I operate.
The most likely reasons for it failing are:
• The "View" permission has not been removed for all appropriate User Groups and on all sub-forums where access is to be prevented. (Remember that, by default, User Permissions are inherited by sub-forums from its parent forum.)
• When testing, you are logged in as a user belonging to User Groups do have access the sub-forum intended to be invisible.
(In fact, they are the only reasons I can think of for the system failing!)
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.