It doesn't matter if that player left the game, if that creature left the battlefield or left your control, or if that creature no longer has a matching type
Keruga, the Macrosage is close to a free roll in some builds of the deck, only costing a sideboard slot
If there's a cost with having that creature attack its owner, you aren't forced to pay that cost, so you may have it attack another player, attack a planeswalker an opponent controls, attack a battle an opponent protects, or not attack at all
And, any 10 or less defense champions that started the turn in play can only trade with it if they block
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Hope any of the above helps