Really not sure. I wrote a couple importers prior to joining Discourse and it was pretty easy, but there was much more prior art to lean on. Someone doing this would be the first to do it for chat, so there’d be more heavy lifting involved.
I think we need to take a “wait and see” approach here. I really question the value of it at this point. Chat is pretty ephemeral and noisy. Does bringing all the old content really help? We are making things default to 90 days retention for the same reason in chat.
Having lived through a migration from one Slack instance into another in the past, I can see how it eases some concerns people may have. And there was a brief period of time when the content was useful. But it pretty quickly became less valuable to have the old content there. I imagine it would have worked almost as well to just keep both instances running for some time, then turn the old one into read-only, then let it disappear.