Was trying to get the hidden artifact appearance for my pally (ret spec) and hit a roadblock. Nefarion’s head no longer drops in Blackwing Lair. As a result, it is now impossible to acquire this appearance. You can still get the other colours related to the ancillary achievements, but not the original one (colour). When I opened a ticket about this, a GM directly chatted with me to confirm that this was a change in 8.1.5. The only way for paladins to acquire it now is for a GM to post ithe head to you via mail.
So just a heads up (no pun intended) to any pally’s trying to get their legion appearances. This is something you’re going to have to do as well if you want the og colour of the ret pally hidden artifact appearance.