Is Unsettled Mariner legal in Standard, Modern, and Commander?

Unsettled Mariner is legal in: Modern, Legacy, Vintage, Commander.

Unsettled Mariner

Q: What type of card is Unsettled Mariner?

Unsettled Mariner is a Creature — Shapeshifter. Its oracle text reads: "Changeling (This card is every creature type.) Whenever you or a permanent you control becomes the target of a spell or ability an opponent controls, counter that spell or ability unless its controller pays {1}."

Creature — Shapeshifter

Changeling (This card is every creature type.)

Whenever you or a permanent you control becomes the target of a spell or ability an opponent controls, counter that spell or ability unless its controller pays .