All civilizations have at least two sets of rules. The two most basic sets of rules are laws and custom, and the most obvious subset of custom is manners. With the recent revival/ renaissance of Jane Austen and various spin-offs, there are a number of writers who focus on manners and social etiquette, generally in such sub-genres as steampunk or Regency-style fantasies.
But all cultures, in all times and places, have unspoken codes of manners, and they’re not restricted to just attire, although at times, cultures have gone so far as to legally define and restrict what people could wear, based on their wealth and social position, through sumptuary laws, which carried significant penalties.
As one of the older practicing and producing writers, I grew up in household where manners and custom were drilled into me. Of course, they had to be, because I was, to put it politely, socially oblivious. The majority of human beings have innate social senses. Mine were largely absent. That made little difference to my parents. I was drilled in every possible social grace and situation by my mother, while my father made certain I was more than adequate in sports, particularly those of social value, while both emphasized the importance of achievement in education. For the time, place, and setting in which I grew up, this was the norm.
What tends to get overlooked by a number of younger writers is that such an upbringing is not an aberration in human cultures, and for the majority of human history, those who have ruled and shaped society have had an upbringing that emphasized what was required to succeed. Those who were well-off but not of the elite also did their best to instill such education and manners in hopes that their offspring would have the background and manners to rise economically and socially.
At present, in the United States, the iron requirements of formality required prior to roughly the 1960s have been relaxed, or battered into scattered remnants of a once-uniform code of elite conduct, just as the former elites have been disparaged and often minimized.
This situation is not usual for cultures. More social rigidity is the norm, just as the studies of Thomas Piketty have shown that, historically, high levels of income inequality have also been the norm. Whether less rigid standards of manners and social behavior are the result of higher technology remains to be seen, but writers should consider [more carefully than many do, and no, I’m not naming names] whether the manners and social conduct of their characters match the actual culture that they’re depicting. The shepherd boy who attains power will never fit [and this almost never happens, except in fiction], except through brute power. His children might, if his wife/consort is from the elite and is in charge of their upbringing.
Also, contrary to what some believe, manners don’t reflect weakness, but are a way of displaying and reinforcing power. The decline of formal manners in the United States reflects the decline of old elite structure, and the often enforced casualness of new up-and-comers is meant as a symbol of a new elite, one problem of which is that an apparent lack of manners too easily suggests a lack of control… and a certain level of chaos and uncertainty.
In any case, any culture will have a form of mannered behavior that reinforces whatever elite governs, something that writers should consider.