Is that needed to make Fossil useful? Since it's basically GitHub but all in Git, it's all works P2P without the need of a centralized service.

I guess for discovery it kind of makes sense, but wouldn't really be "GitHub for Fossil" but more like a search engine/portal.

Also, as far as I know it offers zero CI integrations, which is (IMHO) table stakes for an organization-level platform

n.b. while looking up the modern state of affairs, I found this gem https://fossil-scm.org/home/doc/trunk/www/qandc.wiki#:~:text... which made me chuckle but is also, let's be real, a barrier to adoption. Yes, I am aware of the rabid army of Bugzilla fans, but I'd straight up quit before working with the garbage that is Bugzilla

Discovery and market dominance seems like basically the only thing standing in fossil's way from mass adoption, though.

Depends on how one thinks of "mass," since (a) there is an unholy amount of tooling out there which only speaks the GitHub API (not even mentioning GitLab, Gitea, ...) and (b) AIUI the Fossil folks abhor history mutation which is great for them and really not great for a subset of git users. I get the lightweight impression that they took Mercurial's "please don't" and turned it up to 11