Do the authors have to cite or make comparisons against works on domains such as arXiv?
I think this question is ultimately about standards within particular fields and societies. In math, where arxiv has been central for decades, it would be odd to overlook highly related examples, and using your language it's reasonable to assume authors would be obligated to check. In psychology, where I work, preprint repositories are still fairly unusual, and authors are not expected to review these sources.