This was/is my main gripe with Beyond All Reason (open source rts game) there is no wiki or forums - for an outsider it looks like 98% of all development talk is done in discord.
Though they do have a good basic knowledge base on their website about the game units and mechanics (but I would love dedicated wiki).
I once dm’d the maintainer of an open source project who got kind of upset at me for not posting an issue in GitHub. I got it, it made sense and the guy explained that it was all about visibility.