i can think of 2 popular gaming forums that cater to a specific genre of games
first up is
shmups, a forum for fans of shoot-em-up arcade games like touhou, space invaders, galaga, etc. it's been around for a long time, and for such a niche topic, has quite the active community. they also have boards for general non-shmup gaming discussion, gaming hardware, reviews, meet-ups, and developer resources
i also found
deltarune boards recently, which is exactly what it says on the tin: discussions about undertale, deltarune, and being silly little guys. it's only been around since the end of january of this year, and already has so many members. it is proof that niche fan forums are BACK, baby. move over, reddit and discord!