As of...sometime in the last hour? I can't view threads (or even see most of the forums) in incognito mode, I just get prompted to sign in if I follow a direct link. The Circus of Bug Reports and the Forum Communities are the only ones that show up if you're not logged in. Didn't see an announcement so not clear if this was intentional or a bug.
e: direct thread links are working again now, that was weird; the forums still don't show up when not logged in though
double edit: this behavior is once again manifesting for direct links when not logged in