I remember that back in early Legion days, there was a toggle for the old character models (I don’t like the new models at all, never will) and was just asking if Blizzard removed that option.
Sadly, they destroyed our original characters, and removed them from the game entirely during Legion. You are right that there was a toggle originally, but when they shredded the originals, they removed that.
The toggle was indeed eventually removed, to an extent for actually technical reasons - the new models not just feature higher resolution/polygon count, they also have a lot more “bones” in them to allow animations/posing.
In other words - the old models literally couldn’t have moved in the way some new attacks/abilities demanded.