I had just updated to the newest snapshot, 20w46a, and had completed a blaze farm and nothing was dropping, upon further investigation, I found certain mobs don't drop loot if the world was created before 20w46a. These mobs were cows, rabbits, blaze, horses, squid, chickens, husks, spiders, creepers turtles, and drowned. The ones that I tested that do drop items are puffer fish, salmon, tropical fish, cod, zombies, skeletons, and iron golems. I tested with a sword and an axe, in a world created before and after 20w46a, with and without looting, and got these results.