Will we every see Ensemble: Funerary Plate of the Chosen Dead (Mythic) and other armor types be a boss drop of Helya?
I know it was tied to the title giving achievement and that achievement should stay legacy (like MoP green fire title) however the set should be introduced into the boss loot table for transmog hunters as the other 2 (normal has a quest) difficulty variants of raid finder and heroic have a low drop chance from bosses.
I don’t see anything that says it was removed from the game.
Nothing in that says it was tied to an achievement.
Mythic Version, thats the Normal Version 
Perhaps you could put this up as a suggestion in the Transmog forums? I’ll admit I’m pretty sad I missed out on the Mythic ensemble myself.
I looked further and you are right. You need to do the achievement for it.
If it does get added back, it would more likely be from the Black Market Auction House than as a boss drop.