You don’t choose whether to discard or exile a card from your graveyard until after you see the card you draw.

A card, spell, or permanent is historic if it has the legendary supertype, the artifact card type, or the Saga subtype. Having two of those qualities doesn’t make an object more historic than another or provide an additional bonus—an object either is historic or it isn’t.

You can choose to discard a card even if there’s a historic card in your graveyard you could exile.