A lot of mobs stopped dropping any items at all, (i didn't change any gamerule to stop them from dropping anything). At the moment i know that Enderman, Rabbits, Cows and Creepers don't drop item after death. To confirm i was farming exp in enderman farm for an hour and nothing dropped. I was using looting 3 so this is even worse. But some mobs still drop items, like zombies and skeletons. Only thing that they drop is exp.