Scenario: GitHub, GitLab and BitBucket decide they're just not that into it anymore and are gonna sunset. What are the other cool online VCS communities?

I am building an editor with integrated version control https://github.com/uprun/lisperanto

https://github.com/akkartik/lines.love#mirrors-and-forks Most of these sites are hosted versions of https://gitea.io. Gitea is also interesting because they're working on federation features: https://nlnet.nl/project/Gitea. If they succeed you'll be able to send PRs from one site to a repo in another, stuff like that. (I'm ignoring good questions here about whether git is good, whether PRs are good, etc.)

Gitea federation looks promising! In my little universe of computational science, I see people and institutions move away from GitHub and GitLab towards institutional forges (typically GitLab), making collaboration a real pain. With sometimes surprising consequences, such as me being administrator of a repository which I am not allowed to fork (so I cannot do merge requests).

gerrithub.io uses the stacked commits idea, I think it's based on how they do work at google. Still git based, just need a plugin and a slightly different workflow

What about mercurial? Ive never tried that. Should I? Are there other new models as well?

I used Mercurial many years ago, before switching to git because everyone else used git. There are good an bad aspects to both, but the similarities outweigh the differences in my opinion. Something I am tempted to try one day is Pijul. Looks interesting theoretically, no idea how well it works in practice.

Have any of you tried fossil? I haven't tried it yet, but if the claims on their site are true, it sounds like it's more well-suited than git for most people's personal projects: https://www.fossil-scm.org/home/doc/trunk/www/fossil-v-git.wiki

I am running a small document server plus task management system for my condo as a Fossil repository. Trying out Fossil was one motivation, the simplicity of the setup another one. Fossil contains a Web server and an issue tracker, all in a single executable, so that's all I need to run on my little server. But I haven't used Fossil for code so far. The main reason is that it doesn't have decent Emacs integration. Git has Magit, which is the best UI I have ever seen for a VCS. I'd rather fight git now and then than give up Magit.

Codeberg.org is a non-profit organization that offers a gitea based forge (which looks like github, but is open source). It is getting some publicity and mindshare. They recently forked gitea to create forgejo -- the difference is that gitea is owned by a for-profit company and uses a closed source CI server, while forgejo is owned by a non-profit foundation and uses an open source CI service. It is the Forgejo project that is working to implement federation.