Compare it to a typical Izzet ability like Spellheart Chimera: See how they only pump up with instants and sorceries
Whichever one entered the battlefield most recently "wins." The controller of that Graveyard Busybody has all the graveyards, and any other player's Graveyard Busybody is a 0/0 that is put into the graveyard that player used to have as a state-based action
Its super cheap for the investment, and I sure as hell dont want one of those barreling down at me
Modern: Easily the most popular format among competitive players, Modern allows you to use cards from Mirrodin onward, save for Commander deck cards
One of the recurrent white problems is that white has often less depth on its best effects you can only run one Swords to Plowshares and one Path to Exile, so allowing white to efficiently copy those limited effects gives those smaller number of broken historical effects more presence without printing more broken effects
Near mint, for a rare from Antiquities that is very playable, $30