Forum memberships?

Was this feature removed? I've dug through all the forums and cannot for the life of me figure out how to "join" them. I know I've done it before, since they show up in my profile, like so:

Dyxyi_medium

via i.imgur.com

Which forums require memberships, and which are open? How do I edit which forums I'm a part of? This feature just seems really unfinished to me.