This is a genre of Japanese Fantasy, defined by myself. I noticed the existence of the genre through the consumption of a staggering amount of shōjo manga; concluding it was a genre and not merely a setting… thing.
The genre typically has an Early Modern, pre-industrial setting. I assume the main inspirations would be Victorian England and Absolutist France just before the revolution. This is obvious most of the time, as the clothing and architecture evokes these countries from these particular time periods. The world also has a Western-European climate and geography, most of the time.
Countries within this genre are typically monarchies, where the monarch either rules absolutely or is limited by the aristocrats. Needless to say, aristocrats are usually a big deal within this genre.
Besides that, this genre typically has much lighter fantasy elements. Magic varies from barely existing, to being commonplace, to not existing at all. Fantasy races are typically not present at all and, if they are, do not commonly appear.
Besides the worldbuilding aspects, the focus is almost always centered on the aristocracy and their lives. There is usually a high amount of political scheming and romance between those of different social classes.
Sometimes there’s a sub-plot about the, usually, strained relationship between the nobility and the commoners. This sub-plot appears often enough for me to take note, but not often enough for me to definitely say that it’s a staple of the genre.
The protagonists within this genre are typically female. The background of the protagonist is usually that of a noble lady, or a lady who found herself suddenly thrust into the world of the nobility.
The Shoujo and Josei demographics seem to especially like the genre. I believe this is also why (if Villainess stories are to be believed) this genre seems to dominate Otome games.
The genre also typically overlaps with the Isekai and Villainess genres. In fact, I first noticed this genre through those sub-genres. And actually, I’ve probably read more Isekai-aristocratic stories than non-Isekai aristocratic stories, but that just might be an indication of my preferences more than anything.