I just found out that this is a thing in Arena. You battle for free with starter decks and can earn two rares or better and a random cosmetic.

Go to Home > Play > Events and scroll a bit down. #MagicTheGathering

(Supposedly, you earn starter decks by winning with them. But I already had them since last rotation when they were introduced so I can't confirm. Also, the Izzet and Orzhov decks disappeared from my decks a long time ago. No idea why. #SmallIndieCompany