Under Cooking > Recipes > Old World Recipes, the recipe for Thistle Tea will appear despite my settings specifically disables unobtainables (everything checked) and my class is a mage.

Neither of the two recipes are available to my class (well technically the last one is, but it should be disabled by the unobtainables filter):
https://www.wowhead.com/item=18160
https://www.wowhead.com/item=7678

Both of them seems to use the same spellID to craft the actual item (9513).

This has actually been fixed recently as well as far as I can tell. Though it seems ATT might not have the 7678 Item sourced anywhere currently... so it shouldn't show in the profession list.
Account-wide completed Cooking Recipes on a Monk looks as below:

@KalleZ The recipe is available to every class, but the Thistle Tea item itself is locked to the rogue class.

@DrOxlotus I get that, but the recipe should still honor the unobtainable filter. I don't consider an item (ItemID #7678) that was available until patch 1.3 to all classes a broadly available recipe. The other recipe (ItemID #18160) is only purchasable by rogues.

See the regional availability from TUJ (I play on Darksorrow, so use the regional prices graph):
https://theunderminejournal.com/#eu/darksorrow/item/7678

@KalleZ Since that's the case, we'll fix it.

@kallesommernielsen Upon further review, I don't see Thistle Tea on anything other than a rogue w/o any changes on my part. I checked the main list, as well as the profession list when opening the Cooking frame.

I see it on my hunter.

I was able to enable the account wide mode for receipes and complete it on my rogue alt for this to be marked as completed, but I still think the issue is valid for any non rogues as it should have a class restriction tag in non account wide mode.

Or at least have a new Unobtainable filter in the "No Chance" category if you happen to get one of the extremely rare BoE receipe:
https://www.wowhead.com/item=7678/recipe-thistle-tea