Is there a good forum stack that is as moddable as phpbb was back in the day? Every Discourse instance I see seems _very_ similar so I've assumed there's not much of an interesting plugin ecosystem
It is pretty easy to do plugins and custom themes in Discourse, most look the same just because the UX is good enough and we don't live in an era of hyper customization that early 2000s forums did. A good example of a pretty customized Discourse install is the Blizzard forums: https://us.forums.blizzard.com/en/blizzard/ and Unreal: https://forums.unrealengine.com/categories?tag=unreal-engine
I was looking into forum software maybe 10 years ago and came across Flarum. Worth a look
You could always stay with phpbb - seems it still feels authentic:
https://www.phpbb.com/community/viewtopic.php?f=14&t=2672425
(I'm guessing flarum.org makes more sense, though).
A cursory search didn't turn up any golang based modern and well maintained forums, which surprised me a bit.