They are both walled gardens that you have to subscribe to. Since none of the messages can be seen by the public, they both are knowledge silos. Discord has some wonderful instances with smart people, but by its structure even Usenet threads or old SMTP mailing lists are better to search for answers. I'm sure it is against the Discord TOS for an aggregator to scrape and curate its contents. Unlike even HN, Discord messages have an unlimited time they can be deleted punching holes in timelines. Most of the Yahoo Groups that I follow ended up on groups.io