For Reddit, it depend where you're posting. In my experience: r/gaming and r/Steam are a Big No, they will ban your post or you if you promote. But r/IndieDev and r/IndieGaming are actually good. The trick is to present your game and ask for feedback, include screenshots and link youtube gameplay video and it should pass. Don't spam of course.
Another forum, https://forums.tigsource.com/index.php. I have never been able to get throught their registration process... :-(