I still regularly visit TheGearPage forum for musicians (mostly guitarists), and it's still active.

There's also Hardforum for hardware related discussions, though I don't visit as often anymore.

Many others I used to visit are dead.

Motorsports: forums.autosport.com is very much alive

Also f1technical.net

FreeBSD forums, using XenForo which doesn't seem to be open source: https://forums.freebsd.org/

Arch Linux forums, using FluxBB: https://bbs.archlinux.org/

Free Pascal forums, using Simple Machines Forum: https://forum.lazarus.freepascal.org/

Python forums, using Discourse: https://discuss.python.org/

Nim forums, using their own nimforum software written in Nim: https://forum.nim-lang.org/

68KMLA (68K Macintosh Liberation Army): https://68kmla.org/bb/

electronic music production (hosted by elektron but discussion about other gear is accepted):

https://elektronauts.com

cycling:

https://velocipedesalon.com

https://forums.thepaceline.net

simracing:

https://simracingonline.co.uk

https://www.lfs.net/forum

https://www.overtake.gg/forums/

And a few others I visit less frequently but I won't mention because I have already leaked a lot of information :-)

I guess there are still a ton of forums that cover a wide variety of hobbies/interests.

A small but great little community: https://leviathanalpha.com/

https://www.avsforum.com/ has been running for around 30 years. Still my go to for in depth HT discussion. Much better than reddit.