A legacy forum is what I define as older forum software. Nowadays, people use hip new forum software like vBulletin, myBB, phpBB3 or Invision Boards to make forums. Actually, forums are sort of outdated by today's standards.

There are features on here that reflect some of the relics of the past. For example, in your profiles, you might see AIM, ICQ, MSN or Yahoo.

Nobody uses those anymore.
Those services are dead.