In an almost 'ironic' example, community management doesn't have much of a Reddit presence. The forum administration side is still mostly present on sites like The Admin Zone instead of social media platforms, and the general community management side is more represented by blogs and sites like Feverbee.
ROM hacking is another good example here. Sites and forums about it are doing pretty well (like ROM Hacking.net and SMW Central), but there aren't any active subreddits for it at all. In fact, it seems like the younger generation involved in this hobby skipped Reddit outright, since many of them seem to have migrated to Discord instead.
Fan games in general are arguably the same way too. They've got some pretty active independent forums and Discord servers, but no real Reddit presence so to speak of.
Finally, quite a few media franchises are represented outside of Reddit too. For instance, while it's not as active as it once was, the Wario Forums site in my profile seems to be significantly more active than the Wario subreddit is, with various Discord servers being more signficant competition as a result.
So yeah, hope that helps.