It used to be possible to log in anonymously to the forum.
This feature would hide your name in the "Who's online" list. It would also hide your "last visited" date on your profile.
This feature was present in the form of a checkbox on the login form ("Sign in anonymously").
This checkbox is no longer present (since the forum update ?)
Edit : the option is still present, but it's hidden in the “Security and Privacy” settings
* Step 2: log in
* Step 3: the "sign in anonymously" checkbox is no longer present