From what I recall of the original Bug Report thread…
…there was a period of months when the achievement (and transmogs) were available and then the achievement disappeared for everyone, and the transmogs disappeared for some people but not others. For example, I earned the achievement in July 2021 and thread about this disappearing was from December 2021, i.e. I think that was significantly after the season in which it was achieved had ended.
Indeed, since the last update we got from Blizzard was also in 2021…
As they didn’t fix it in the last three years, I can’t see them doing so now.