You ask the impossible: Finding classic world-drop recipes

I've been running a Challenge Modes 101 series recently to help players who are scrambling to get their transmog sets, mounts, and titles before the axe falls, and our commenter Drewbob pointed something out that I hadn't considered. The use of Invisibility Potions is a key part of the strategy for gold timers, but the recipe is a world drop in classic Azeroth. If you weren't lucky enough to get a drop while leveling, or buy it when it was more commonly available on the auction house, you may find it very difficult to get these days. You can't really farm for it, and on smaller servers, you've got to wonder if it's ever available at all. I bought my Invisibility Potion recipe back during Burning Crusade, and now that Drewbob's mentioned the sheer difficulty of finding it these days, I'm so grateful that I was always too lazy to switch professions.

Don't get me wrong -- it's actually a lot of fun to track down rare recipes all over the world, but the world-drop model might be a little too punishing these days. Would it be better to make world-drop recipes, say, a rare drop off a specific set of mobs? Or moved to vendors linked to old-world reputation grinds like the Thorium Brotherhood? I feel like that would accomplish the goal of keeping a particular recipe less common while reducing the insanity of finding the proverbial needle in a haystack. It might also make it easier for players to change professions without the threat of losing recipes that they might never find again.

Out of curiosity, what other profession recipes are world drops that are tough to find these days? I'm not too familiar with anything that's not alchemy, and maybe we have it easy compared to some.