If someone said to you, an informal code of conduct among European lower nobility 900 years ago dictating, not just court manners, but a warrior's ethos – well, I have terrible news. It's dead.
You might reasonably respond "... yeah...?" – nobody would be surprised, I think. Or care.
I think there's meaningful work that goes into any system of etiquette, no matter how baroque, or rooted in a world we no longer live in. But bits survived for longer than others. America proudly has no court, no nobility, no lords or ladies. So court etiquette was out a while ago.
Although my scholarship on this is weak, I suspect a warrior's ethos survived much longer. The wording of the 2nd Amendment, for instance, hints about a social expectation of ordinary men taking up arms as a part of their duty to their neighbors. I'm not sure when that went away. Maybe after World War 2. No war since was ever quite as worthy as that one, not for Americans, at least. Just minor battles or proxy battles, more abstract things or places you'd never heard of.
America recently lost a lot of its religion. That was a big part of chivalry.
But when people say "chivalry is dead", they're not talking about their combat readiness, their religion or protection of the church, or even court manners – they're are talking about (please excuse my careful wording) a sexually asymmetric standard of generosity.
Knights would not have noticed this, I think – perhaps because women weren't knights and wouldn't be bound by the rules of chivalry? So it wasn't a man-woman thing, but a sign of their lesser nobility.
I think the real question here is, what's the new standard? If I'm not to be a knight, what is my role? I try to be a good person. I succeed sometimes. I try to be honest, which is much easier. Generosity is harder than it looks, but I'm getting better at it (it isn't, as you might think, purely about setting aside money for gifts, although that isn't a ridiculous place to start).
The real odd part is that "chivalry is dead" points to a cultural gap – there's no code of excellence, no real role models anymore. We'll find them, at some point, and they'll be different (it's a very different world). But in the meantime, noticing that gap seems important.