Because I don't moderate and don't really engage in any communities I never really considered this question.
I am certain it varies quite a bit across games which makes me think it's just up to the whims of the game's community or an artifact of a game's speedrunning evolution.
From what I see they include anything from old any% times from before skips were found almost like an archive, joke categories, fun stuff that people want to compete in but aren't considered serious and fulfilling abitrary criteria the game tracks but doesn't "finish" the game.
Wouldn't anything aside from "beat the game as quickly as possible without external means or cheats" be "miscellaneous"? I have seen leaderboards where categories that get enough competition are added and not considered misc.
I'd be a little surprised if there was a consensus, I'm just interested to see what people think or other communites us them for.
Kinda depends on the communities and the games tbh.
Usually;
- Old routes
- Joke categories (if there's enough competition these might not be in "misc." anymore, usually)
- Sometimes other things (extra dungeon you normally can't do with just one cartridge, in boktai's case)
When people come to SR.com and take a look to leaderboard, it's not very productive and useful to show on the main page of a game all leaderboards. So the solution is to keep on the main page the leaderboards/categories that represent the community the most, and put the rest on miscellaneous category. So some categories go to misc because they were popular once, but not anymore, or the game has evolved so much that runners don't do performances on old patchs, joke categories...
This is something generally decided by community and mods, so sometimes, couple of categories can go back from misc to main page, when the majority of them wants to.