Is Cubwarden legal in Standard, Modern, and Commander?

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

Cubwarden

Q: What type of card is Cubwarden?

Cubwarden is a Creature — Cat. Its oracle text reads: "Mutate {2}{W}{W} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.) Lifelink Whenever this creature mutates, create two 1/1 white Cat creature tokens with lifelink."

Creature — Cat

Mutate (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)