there’s an entire branch of object-oriented language design without classes!

That's not OOP anymore. There's definitely a lot of OOP code out there (especially in the Java world) that goes way too far with inheritance and class structures, and good OOP code relies a lot less on classes - but they are still used and a critical component of the style of programming.

Object-oriented design is about message-passing;

I'd argue that's an implementation detail rather than part of OOP. Also it's rarely used in modern OOP languages because it's just too slow*. Unfortunately when you take it away some patterns are lost, but the trade off is generally worth it.

(* when I used to work in Objective-C, message sending was often slower than all of the rest of my code combined and in tight loops I'd often rewrite my OOP code as procedural C code in order to have acceptable performance. Never need to do that in Swift, which doesn't do messages)