Honor is a fairly amorphous concept, and it is hard enough to even agree on a real definition in its study. From an anthropology perspective, there is a lot of work on this which really takes off in the 1960s under Julian Pitt-Rivers and his study of honor in Mediterranean societies, and more recent works such as Frank Henderson Stewarts Honor and Anton Blok's Honour and Violence build off of, and at times push back against, Pitt-Rivers' work. I write this preface to offer some angles of investigation but also to caution that it is hard to talk about honor as a unified concept.

Now with that said, honor as specific to my own studies and tied intimately to the duel is a bit more defined, but even then, hard to see a single, unified narrative of decline. I've written about several angles though which I would point to, with at least a broad overarching theme being that the decline of honor, at least as a quality which defined a gentleman to the point were one was expected to lay down ones life to defend it, was often, but not always, tied to the swift and cataclysmic social changes that followed major conflicts of the 19th and 20th centuries, which tie to the decline in almost all major dueling countries (France, Germany, America, Russia, Italy) except for the United Kingdom. If not the focus, I touch on this matter in several answers which I have written on dueling, which can be found here Off hand, I think the most in-depth piece I wrote is this one, which is a follow-up to a different one, but fairly comprehensive. If you want really in-depth, I have a long list of recommended readings here. I'd draw particular attention to the works of Bertram Wyatt-Brown, however, as I consider him the absolute best author for ideas of 'honor' in 19th century America.