>>So, no more classic ships, just mobile blocks of flats.<<
What is "classic?" As with anything else in the shipping industry, form follows function, and the "Classic" edwardian liner we're familier with was the end result of design concepts and requirements which only came into play during the latter part of the 19th Century. "Traditional designs" before that were in sailing vessels with masts, yards, sails, long bowsprits, figureheads and the like which were great for wind driven vessels which had dominated for thousands of years but which were of little value for a steamship. The long bows of the "classic" liner we're familier with existed in no small part due to the fact that these ships also carried cargo and the mails. The rapid increase in size came about because of the demand for not only more accomadation for the passengers but better and more roomy.
The same considerations for passenger vessels apply today. Passengers want more room, better and more comfortable accomadations, and external rooms with a view. They also want shopping arcades, theatres, casinos, swimming pools, rock climbing attractions and even putting greens for a game of mini-golf. What you don't see is a need for space to carry cargo and mail, and that's why you ssee the floating condos of today as opposed the combination cargo/passenger vessels from the days when it was literally the only way to cross.
On the matter of what's illegal, the sort of bow that the Titanic had would be a no-go because it would act much like a knife in a collision. A long sharply raked bow just doesn't penetrate that far. Flammables would also be an issue. You won't see bedsheets and blankets made of asbestos or nomex but all the heavy woods used for the panaling in the old liners would have the authorities screaming foul.
Beyond that, you may find that in basic form, there's not as much about the older ships which would be illegal in and of itself. If somebody wanted to design a hull with the same percentage of space set aside for cargo and passengers as the Olympic or the Queen Mary, they could. (Just don't count on seeing it happen since the cargo space would simply be unused and wasted.)
Hell, the Olympic class's watertight subdivsion and floodable length curves are way superior to what a modern cruise ship has. No problem being legal with that arrangement.
Show me any modern cruise ship which could remain afloat with up to four watertight compartments flooded. An Olympic class liner could. If you flood more then two on a modern cruise ship, I hope you're up on your swimming qualifications.
