From the
Battletome Legions of Nagash Errata:
Pages 117, 118 and 119 – Cairn Wraith, Tomb Banshee, Hexwraiths and Spirit Hosts
These warscrolls are no longer used. They have been replaced with the warscrolls in Battletome: Nighthaunt.
Those units are still in the Legions of Nagash battletome, they've just been updated to match the warscrolls in the Nighthaunt battletome. As for the other units (e.g., Myrmourn Banshees) I see no indication that they will be added to the Legion of Nagash battletome.