Have you actually tried to search for items that disenchant into astral using wowhead classic? You’ll see that you cannot find any. Most of their info is populated using user contributions as well.
Try using the “Disenchants into” filter and you’ll see that astral essence is missing.
Try search for items that SHOULD disenchant into astral essence and you’ll see that magic is in it’s place.
Here’s a few more example of items that should disenchant into:
https://imgur.com/a/BrfVjCQ
What you’ll see in the image are weapons that are disenchanting into Greater Magic Essence instead of Greater Astral Essence, and when purchasing a weapon that moves the enchanting material into the next tier, it goes back to normal.
So tell me, what happened to a whole tier of disenchanting material?