I was just checking things on the PTR and I just noticed that The Chosen achievement and title are still available on the dungeons and raid tab, whereas the challenging look achievement has been moved to the legacy tab.
I don't know if this is final but I just wanted to let everyone know because many people are paying lots of money/gold for that and it seems, at least for now, the achievement is here to stay
Edit: Seems like the achievement will be moved to a FoS soon and be unobtainable https://twitter.com/WarcraftDevs/sta...45805501837316
They haven't said anything about the mythic set linked to the achievement though.