With how many times I have seen the "forum" for a community be a Discord server I would say that there has been a noticable shift away from independently hosted forums. Which is what I would associate with a "proper" forum.

This, I don't really understand this "bring back" things, there are a handful of forums I am visiting almost daily. And they are probably my most visited websites alongside hackernews and a couple of others.

Such as?

https://tl.net is a great old school forum if you like RTS games

Could you list a few?

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.

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

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/

LinuxForums has been going strong for decades. My account there dates to 2004.

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

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

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

Bogleheads forum is great: https://www.bogleheads.org/forum/index.php

There's a Spain equivalent forum also if you poke around

True, but I genuinely think we lost something over the years. This all might be a feeling that we lost what was, and that we know we can do it again, but it's impossible to replicate what we had (format and, more importantly, the users).

It might not be a platform issue, but a "corporate X decide to scrap forum Y", and it never was the same again.

I've seen this a few times, and it basically blew up incredibly tight and excellent groups.

Yeah, ideally the system used should be something distributed like NNTP or a free software forum software with an easy import/export option + frequent export snapshots anyone can download.

Then if the forum goes down for whatever reason, one or more community members can rehost it elsewhere.

Even there, though, a lot of the forums are dead or decayed or have disappeared. It's not like it was even 10 years ago.