Yes, there are some forums on the board which aren't visible to some people (and are never visible to non-members or members who aren't logged in.)

Are You There Free Jinger, It's Me Margaret aka AYTFJ:  This is a forum for all members who have at least 75 posts.   Content here is not submitted to search engines, but it is still the responsibility of every poster to remember that the internet is forever and amassing 75 posts isn't a high bar to clear.  Site rules prohibit copy/paste and direct quoting of posted from restricted forums.

Please remember that this is the internet and it's not rational to have an expectation of privacy on a forum with 10k + members just because the newest of which (and non-members) cannot view the content.

Other forums:  There are technical and administrative forums to which members get on a need to access basis based on their roles in admin, tech, helpmeets, etc.

*Active = logging in.  Not everyone posts a lot and so this rule was changed slightly for the quieter members of the FJ community.