So, as the topic describes, my achievement disappeared. It is no longer there.
I logged in the day before the season ended and grinded out the ethereals I hadn’t gotten before just to get this achievement. However, to my great disappointment, I now do not have it. (I thought about taking a screenshot but I figured there was no reason…)
I would imagine the achievement would be moved to “feats” since it is no longer possible to obtain.
To clarify, feats were added into the achievement list since RoS was introduced and the Normal–Nightmare–Hell–Inferno progression was removed and thus achievements tried to those difficulties were now defunct/impossible. The feats count as zero points towards achievements and are a simply a way to record legacy achievements.
With regards to the Ethereal Recollection achievement/feat, the main bonus is the ability to transmog your weapons into those forms. If you have those transmogs, you should have the achievement (maybe viewable once Blizzard updates the list).
I actually saw a video in which someone had the FoS in their “career” tab and not their “season” tab. I figured that it would move over once the season ended as Iria mentioned, but it did not.
I do not have access to the Transmogs either. If that is a common issue I will wait for an update.