I'm struck by the fact that for centuries there were complex rules of etiquette established for interacting with other members of society depending on class, gender, family relationship, etc. Then during the 20th century that formal system of rules was all but abandoned.

Were they really? Here in France, when you meet a woman you kiss her on the cheek, but when you meet a man you shake his hand; you use different pronouns ("vous" or "tu" - cognates to "you" and "thou" in English) depending on the relative status of your interlocutor (and other things); in many western countries (the US more than France; though it seems) it still seems expected for a man to buy an overpriced piece of rock to the woman he's planning to marry and not the other way around, etc. - we have plenty of rules that depend on gender! (probably more than on class)

I think that what happened is that there was an effort to increase fairness by removing some discriminating rules, which meant those rules became weaker, but also more likely to be tacit: since Victorian society didn't consider gender equality to be a major principle, there wasn't anything wrong with spelling out the norms that regulated gender relations (unless they went against other values of the time). Now nobody wants to sound sexist; so people have to figure the rules out on their own.