Many no longer obtainable appearances from Naxxramas are being made available again in Dragonflight Patch 10.1.5, using Ancient Plans similar to the return of Zul’Gurub cosmetic collections in 10.0.7!
In a group interview, Game Director Ion Hazzikostas recently commented on the intent to add previously removed cosmetic appearances, with the exception of very specific notables like Ashbringer.