It has been done before lol. It can be done again. Also thanks to patch 2.1 and above, it's so much easier. The hard part is the effort to do so and then the Eternals with crystal affixes. Eternal codex is only one part of the challenge (I remember I had to note things down before eternal codex).
Well centrabpm done collect all Eternals of that time except for one eternal of all classes before Ascensions until new legends come out but not including Eternals with crystal affixes. Nowadays these are much easier to do of course.
But yeah it will take a while. I can definitely do it provided I'm not lazy to check my screenshots or farm build like I've been for a while. Although I used to be not lazy.
Yes there is eternal pets hunting as well but that's not included due to the super probabilities involved but getting many eternal pets helps.
The easy part is doing it for one class. The hardest part is doing it for all 3 classes and making builds for them and actually doing it.
I know dust exists but yeah . Still, I gotten quite decently far. Only reason I even thought of this challenge was thanks to patch 2.1 Ascensions or so. From 650% gold and luck default to something like 1012% nowadays. Huge difference and of course the item drops are the same.