Closed, by itself likely means the thread somehow got out of hand and was either temporarily as a cooling off period or permanently if it was totally out of control. It can also be closed by 2 weeks of inactivity, or if it got too long… for instance 1500 or 2000 posts, although that could be 1000 posts in some cases.
It could be unlisted if the mods found something really offensive that they feel should not even be viewed.
The terms of service and forum guidelines are published on the site. The 3 horizontal bars button will eventually get you to them. Among guidelines is no: spam, singularly forcing a point, ad hominum attacks, uncharitable comments, etc…