Eighteen-inch guns, Furious, and hypothetical modern armor

Would Yamato's 18-inch shells be useless against armor made with modern metals?

No; they are still considered viable, while the related discussion emphasizes gun design, barrel length, and the limits imposed by the carrying hull.

Yamato's 18-inch shells are still considered viable in the hypothetical contest with improved armor. The explanation then turns to the British 18-inch guns fitted to Furious, whose 39-caliber barrels are described as constrained by the hull. A Queen Elizabeth-style battleship hull prompts speculation about a longer weapon of 42 or 45 calibers. That would create a very large barrel, with corresponding implications for fitting and moving it aboard ship. The later discussion explicitly leaves the unbuilt Agincourt design uncertain, so the suggested barrel lengths remain possibilities.