I believe it’s available as a reward for legend, but you can also get it from lost sectors. I think the only campaign locked exotics are from shadowkeep.

Need to complete Witch Queen on Legendary first. Then Ikora offers a choice of Loreley or Hoarfrost, but the rolls are sh-t so it's best to pick Hoarfrost. After that, can farm lost sectors. If they're rewarding head armor, Loreley should drop pretty quickly; after which, it'll continue to drop randomly at the same rate as other armor pieces.