When you join a forum remember that you may be there until the forum is removed, which in some cases could be many years if ever.
Some forum software will delete all threads and posts by a user if that user account is deleted. So it seems that some forums will not delete a profile even at the users request.
Therefore if you are joining a kink based forum it would be sensible to use an alias as a username. If you use your real name or identifiable details then you better make sure that you would be happy for your presence to be found many years later.
Forums grow organically and changes can happen to focus and ownership which can change the feel of the place you thought you knew so well. Even my forum has changed as my experiences have widened.
Something to consider when joining a forum is that your presence may never leave, threads you started some time ago may be dug up and their focus changed as others put their slant on them.
So what legacy will you leave on a forum? Something worth thinking about.