In August last year I got this from dang when reporting a dead 404 link: "The site 404media.co is banned on HN because it has been the source of too many low-quality posts and because many (most?) of their articles are behind a signup wall."

Not that I've really seen the low quality and the signup requirement doesn't stop other domains. There's quite a few things that originated from 404, so I hope HN gets over whatever it was that annoyed them originally.