may be it's to avoid search engines to not scrape these threads?
My understanding is that it was causing support problems, because people were Googling for solutions to problems with their apps (because of the Heroku diaspora, we have a lot of first-time Docker users), finding old stale threads on our forum that looked related, and then reviving them.
I think we can just `noindex` the category instead of making it private?
So the tagged posts were intentionally hidden, then.