This is more a post for my brother's situation, but he asked me for help and I don't know the answer to it either so I'm turning here. He did the first mission of forsaken as well as the first one on the tangled shore way back when Forsaken was still the main dlc at hand. He hasn't really played destiny since (I'm daily-weekly player, hence why he asked if I knew why) especially the story. Now that the destinations and everything have been unlocked with shadowkeep and new light, any progression (such as hunting the barons) is gone for him. He plays on PS4 if that helps in any way. Thank you.
Progress was reset for all legacy story campaigns when New Light launched if they had not been completed prior to October 1. Your brother should be able to restart the Forsaken campaign by selecting the first mission node at the bottom of the Tangled Shore map.