Not sure if I worded this properly. When choosing to "move" a thread to another forum, you are given the choice of forums that are not available in the node tree which I call closed forums. If one chooses to hide, close or not show a forum then it should not be a choice in the drop down menu.
Forums invisible in the node tree? I use that to have archive forums, it's faster than editing permissions imo.
I also use this option to create new forums for an update. I keep them invisible that way but knowing the address, I can still use it and again, it's faster than editing permissions. When I'm done, I just display it in the nodetree and I'm fine