Is Rattleclaw Mystic legal in Standard, Modern, and Commander?

Rattleclaw Mystic is legal in: Modern, Pioneer, Legacy, Vintage, Commander, Historic.

Rattleclaw Mystic

Q: What type of card is Rattleclaw Mystic?

Rattleclaw Mystic is a Creature — Human Shaman. Its oracle text reads: "{T}: Add {G}, {U}, or {R}. Morph {2} (You may cast this card face down as a 2/2 creature for {3}. Turn it face up any time for its morph cost.) When this creature is turned face up, add {G}{U}{R}."