The community I follow most closely is the Abstract Games subforum on Board Game Geek. BGG's defitinition of abstract games is broad, but the people who hang out in those forums are mostly concerned with two-player combinatorial games. The community is small, but some designers are quite prolific.
The games are non-commercial by nature, but not necessarily by intention. Every once in a while someone comes in asking if it's possible to make money from abstract games. The answer is no, and exceptions are far too few. Money aside, some folks try to design for mass appeal, for pleasant first experiences, which has led to heated debates about how that affects the design in possibly bad ways.
But yeah, it's mostly weird games in a genre that's not entirely unpopular (see chess), but the people who play them prefer to focus on one game (see chess).
What else? I used to read the FreeGameDev forums, but I'm slacking hard (hi ffaf?). I've been playing Space Station 14, one of Space Station 13's successors, so that's underground, right? But most of the community seems to gather on Discord, and I don't do that. I also read The Trick-Taking Guild on BGG, but at this point I start wondering if that's underground enough. The whole mOdErN bOaRdGaMeS sCeNe is not exactly mainstream, but it's VERY commercial! But yes, there are people making new games for ye ole deck of cards, or something else available as print-and-play. And I cannot, for the life of me, place those boutique Japanese card games on the commercialness scale.
Oh, quick edit: Gemini is fucken cool! But I haven't read anything in there in ages, and I have yet to put up my own capsule...