Is there a cruise line that does not require a jacket and tie anytime anywhere?

Oceania, Windstar, Star Clipper, SeaDream always, RSSC on most all of their cruises.

NCL is a less costly option.

All of the mainstream lines offer optional casual dining on each night.

Of the mainstream lines (Carnival, RCL, Princess)only NCL never requires a jacket and tie anytime, anywhere.
As listed some of the high-end lines do not require jacket and tie.
On Carnival, RCL and Princess you may dress casually at the buffet.

On our recent Norwegian Cruise Lines trip to Alaska, we couldn't been more informal without having the police called in.

We have taken the Windstar sailing ships and most recently the Regent (formerly Radisson) Paul Gauguin and neither one requires jackets. No jeans in dining room but no ties either. Some people had them on but we more tropical clothing. Some people I know who have taken the more formal cruises, often have room service when there are "formal" nights.