I used to run a php board in the late 90s I think. It needed to be constantly watched and updated otherwise it was spammed and hacked horribly. I didn't really have the skills and time to keep on top of it. Having a reddit group would be way easier, but reddit is kind of annoying now. There is zero technical knowledge and minimal time to set up a Facebook group and almost all not for profit groups seem to use Facebook for this now in the UK at least. This is actually quite a good feature of Facebook IMHO.

I do know a few sites that run custom bulletin boards for my hobbies, but that requires a developer to be available much of the time to ensure uptime. 2 examples are:

https://www.ukriversguidebook.co.uk/ - which is originally based off phpbb I think

https://www.ukclimbing.com/forums/ - which I think is completely custom