Is Stern Mentor legal in Standard, Modern, and Commander?

Stern Mentor is legal in: Modern, Legacy, Vintage, Commander.

Stern Mentor

Q: What type of card is Stern Mentor?

Stern Mentor is a Creature — Human Wizard. Its oracle text reads: "Soulbond (You may pair this creature with another unpaired creature when either enters. They remain paired for as long as you control both of them.) As long as this creature is paired with another creature, each of those creatures has "{T}: Target player mills two cards.""