Ask HN: Is there a new and interesting open source forum library?

Ask HN: Is there a new and interesting open source forum library?
4 by el_dev_hell | 0 comments on Hacker News.
There was php BB in the early 2000s, which still seems to be active (but very dated). Discourse is nice, but it's certainly in the stable phase now. Is there anything new/interesting happening in the open source forum software space? I'm mostly asking out of general curiosity to see what's breaking new ground.