Restricted Items is a type of glitch which allows you to use items or buttons in situations where they are supposed to be dimmed.
Simply play the Day 3 Honey and Darling minigame as deku link and you will be able to use any item on C.
If you get on Epona, enter a loading zone, then save at an owl statue and reload the save, you will be able to use any C button until you pause, leave the area, or transform to Deku, Goron, or Zora. Since the glitch gets cancelled when transforming, if you want to use restricted items as a transformation, save at the owl statue again.
This method uses the state restoring effect of the 0th Day glitch to keep buttons undimmed after switching forms.
Get Ledge Down A then equip Lens of Truth and climb on a ladder to enable a c-button. You can then equip something else over the c button to use it. This can be done on platforms which go underwater such as the ones in Great Bay Temple. It can also be done on the icebergs in Mountain Village.
Explode the Blast Mask then climb on a ledge twice. Get a ledge popup and you will have 1 frame to use a non-cutscene item. You can also do this after a cutscene to allow using a cutscene item for 1 frame.
You can get Down A with a ladder, then equip lens of truth to enable a C button. Then get a ledge popup and you will have 1 frame to use a non-cutscene item. You can also do this after a cutscene to allow using a cutscene item for 1 frame. This method is shown in the video above.