While all the info can be found on the Drops section of the wiki, it is a hassle to check the wiki for each drop that you are looking for. Personally, I'm constantly checking the page. It would be simpler if there were a way to simply check in-game. Drops often change and the changes to drops aren't always indicated on the changelog. This command would also help if a server decides to change the drops.
/checkdrops <item> [biome]
If I were to run the command /checkdrops shulker_shell, all the Pokemon that can drop shulker shells would be displayed in chat. If I were to run the command /checkdrops diamond nether_wastes, all the Pokemon that spawn in Nether Wastes and can drop diamonds would be displayed in chat.
It also would be nice if the chances of the Pokemon dropping that item would get indicated somehow, either by displaying the percentage next to the Pokemon's name or changing the color of the Pokemon's name depending on how common the item is (green means 50% or higher, red means 10% or lower).