In 21w15a, the ore generation (save copper) was supposedly reverted, however, it doesn't seem that emerald ore was fully reverted, as I found no emerald ore is spawning in the extreme hills biome after a large amount of searching. In 1.16.5, emerald ore spawns with relatively high frequency at Y=11 in the extreme hills biome, however, I used a /fill command repeatedly to remove all stone and such blocks (base stones). I thought that maybe emerald ore was accidentally listed as a base stone, however, I checked, and it is not.

After using a command to clear away the stone, I found emerald ore generating at Y 79, which did not occur in 1.16.5. This means that emerald ore was not properly reverted.

The command I used was:

/execute at @p run fill ~-15 ~ ~-15 ~15 ~31 ~15 air replace #minecraft:base_stone_overworld