From what I've seen, communities don't migrate; they fragment. It's very hard to get a group to move, even if the new option is clearly better in the ways that matter to the group.
When I left Stack Exchange I joined the Codidact project, which is FLOSS and run by a non-profit foundation (disclosure: I'm on the board). We still have lots of things we want to improve (we're a very small team), but probably our biggest challenge is adoption -- attracting enough people who want to do Q&A and related knowledge-sharing somewhere that puts communities and people first and isn't driven by revenue goals. We've got communities for software development, Linux, and others, and I'd love to see them grow.
They still sell Stack Overflow for Teams (renamed Stack Overflow Internal or something like that), but the cost is pretty astronomical. If you want private Q&A in your company/school/etc and you've got anybody with the tech clues, you're better off downloading and setting up one of the free tools.
We outlined some of our broad goals (intended differentiators) here: https://meta.codidact.com/posts/276296, in case that helps. Codidact is a work in progress. The biggest non-technical difference from SO is how we treat communities and their members: communities have a lot more autonomy, and we treat people decently. No stockholders are driving anti-community business decisions.
On the technical level, while Q&A is central, we also have other post types and other models. That post I linked to is an article in a blog that's part of our Meta community. The Electrical Engineering community has papers, so people can present information outside of the Q&A structure. Code Golf has a sandbox where people can get feedback on draft challenges before posting them. Software Development has a Code Review category. Some of our communities have added their own customizations to the code, like Code Golf's leaderboard for challenge answers. We want to work together with our communities to build what best serves their needs.
We've done some things that look small but might have larger effects. For example, the asker of a question can't mark one answer as "accepted" like on SO, but anybody can mark an answer as "works for me" -- or "outdated", or other annotations that communities can define. Scoring takes controversy into account, because +10/-5 and +5/-0 are very different even if they're both "net 5". With threaded comments, it doesn't matter so much if two people have an extended conversation; it's not in the way. Abilities are granted based on activity and reputation is just a number -- or can be turned off entirely if that's what a community wants. We're trying to make as much stuff configurable as we can, because we can't possibly know what's going to be best for every single community and don't have the hubris to claim we do.
We have the usual bootstrapping problem of a new thing. Our communities are small and trying to grow. Because they're small, visitors don't see thousands of questions and high activity, so they don't participate either and wander away, making it harder to build activity. We would love to find people who want to work with us to build communities. We recognize that helping to build a community with us is going to be harder and slower than just asking your question on SO, but if everyone were happy with SO this thread wouldn't be here, so maybe we're an option to a few people reading this?
(I haven't posted much on Hacker News, so I hope I've read the room correctly and that this kind of comment is ok. If not, I apologize and would appreciate correction so I don't repeat mistakes. Thank you.)
Codidact hosts a network of communities, so if you've got a group of people who are enthusiastic about a topic, you can propose it instead of having to host it yourself. Of course, you can also download the code and set up your own, same as with Q2A or Discourse or others, if you prefer to host.
There are differences among all of these platforms (and Stack Exchange) in terms of specific features. I'm not as familiar with Q2A as I am with SE, so it's hard for me to do a detailed comparison, but some things that I think improve Codidact include: more post types (like articles), privileges based on related activity rather than overall points (for example, a pattern of good edit suggestions leads to being able to edit directly), categories, and lots of customization options for communities.
When I left Stack Exchange I joined the Codidact project, which is FLOSS and run by a non-profit foundation (disclosure: I'm on the board). We still have lots of things we want to improve (we're a very small team), but probably our biggest challenge is adoption -- attracting enough people who want to do Q&A and related knowledge-sharing somewhere that puts communities and people first and isn't driven by revenue goals. We've got communities for software development, Linux, and others, and I'd love to see them grow.