Why modern warships still need strong decks

Would an armored or splinter-protection deck be useful on a modern large warship?

Modern ships already have strong structural decks, even when those decks are not classified as traditional armor.

Large modern warships have not abandoned protective deck structures altogether. Every deck contributes to the hull's strength and uses material capable of carrying structural loads. That does not make each one an old-style armoured deck. The distinction between armour and structural protection needs care when comparing eras.