Mushroom

The mushroom is a Y-item in ALttP. It has no intended use beyond its involvement in a side-quest for the magic powder. It has a glitch usecase with fake powder. Outside of these two things, it has no other real utility.

Location

The mushroom is found sitting on the ground in the Lost Woods. It can be accessed and picked up immediately at the start of Zelda rescue game state and doesn't require any items.

It can also be accessed during the rain state in Skeleton Forest.

Magic powder quest

The mushroom exists to be exchanged for the magic powder. You're meant to give it to the witch stirring the cauldron on the witch's hut screen. When you leave the screen and come back, you can enter the shop and magic powder will be sitting on the ground next to the shopkeeper. Magic powder replaces the mushroom's slot in the inventory, and the mushroom can't be recovered after turning it in.

Fake powder

Outside of the magic powder quest, the mushroom is notable for enabling fake powder, a glitch where the player can use powder without actually having it in their inventory.