They’re often desperate communities based around specific hardware. Like the Amstrad CPC, BBC Micro, Atari ST or other retro computers. Plenty in retro consoles too.

I’ve not found a single forum where people lurk but rather independent message boards. I believe there’s a few Facebook Groups, possibly Redit too, but I don’t personally use Facebook nor Redit. You generally find them when looking to do something interesting on a specific device.

However if anyone knows of a more centralised place then I’d be interest to know too.