I've been looking around in the rules but I don't see any restrictions on posting adult content, or even the option to mark it as such.

Since I make adult games/art, I'm curious ... does that mean I'm allowed to just post in say https://itch.io/board/10095/2d-art?

Because there's no option to mark NSFW content, please be careful what you post in the forum. If it's explicit, post a mild or censored version, and link to the page of a project marked as NSFW, where you can safely place the full content. Thank you.