I don’t, though I’m not an admin for any of the lists I’m dealing with. But migrating the messages would appear to be addressed by discourse/script/import_scripts/yahoogroup.rb at main · discourse/discourse · GitHub and GitHub - jonbartlett/yahoo-groups-export: Export Yahoo Groups forum to file or database · GitHub. And a group admin would be able to export the group messages with the email addresses attached (which a normal user can’t). Although it doesn’t look quite pushbutton simple, the messages appear to be the least of the concerns at this point.
Yahoo groups provide (well, provided) storage space for photos (100 GB) and other files (2 GB) for the group. Groups I’m in use these for pictures of group members and items of interest to the group, and for various other files. Anything that would be sent privately among members in a Y group would likely have gone as an email, and Y wouldn’t have any record of that; I wouldn’t think migrating that would even be possible, much less a priority. But there is a lot of information stored there in many groups, which they’d want to preserve in a migration.
Could be. Again, there’s one (and only one, AFAIK) site that appears to be a turnkey replacement, but it maintains the old format of Y groups with only minor changes. I’m thinking that, if a group has to migrate anyway, it might be nice to migrate to something more modern, and Discourse still plays pretty nicely with email (which many other forum packages don’t), meaning that the old-school folks like me who are used to getting the email delivered to them, and replying by email, can still do it that way. And saving a few bucks would be nice as well.