Forbidden Forest and similar games are like the great grandparents to games like Shadow of the Colossus that are just boss fights which I just love. Forbidden Forest is legit very tense and I love it to bits which is why I find it sad that is not run at all here. Only one person has submitted a run for it and that was 10 years ago. I wanted to submit a speedrun of it just for fun and it will never be published because the person over it is inactive.

So how do y'all go about getting a community for an old niche game like this started?

it will never be published because the person over it is inactive

If you mean that the moderator won't verify the run, then you could submi a Support ticket, and request to become the moderator.

Anyway, I think that normally games become active in speedrunning when they are a popular game beforehand. Also this game might be fun to play, but maybe it's bad for speedrunning. For example, if you can't optimiste it very much.