Question

What are all the instances of ??? type?

Answer 1

Generation 2: Curse
Generation 3: Curse, all Pokemon Eggs' typing
Generation 4: Curse, unused Arceus-??? type
Mystery Dungeon R/B and Explorers: Wide Slash, Vacuum Cut, unused moves Excavate and Spin Slash

All Shadow moves in Colosseum and XD, including Weather Ball in Shadow Sky, are considered typeless, as well as a pure Flying-type using Roost in Gen 4. They are NOT considered ??? type.

Source: Bulbapedia
Hope this helped!

Answer 2

??? Type only was in Gens 2, 3 and 4. In Gen. 2, 3 and 4 Curse was a ??? move.
In Gen. 3, Eggs were displayed as a ??? Type to show that the type was unknown. The Final appearance was Arceus in Gen 4. The y added the sprite to likely accommodate for hackers. Note these are only main series games.
Source: https://m.bulbapedia.bulbagarden.net/wiki/???_(type)