The account export feature now lets you specify which groups you'd like to export data from. There's also an option labeled 'Any other groups' which exports data from any groups you may have posted to in the past and later unsubscribed from. I have also broken out the messages by group into separate mbox files in the archive. Please let me know if you see any issues.
These things are still up in the air because every lawyer involved with GDPR stuff is slammed right now and getting time with my lawyers has been challenging. I'm doing what I can, but my guess is that these changes will happen after Friday. I've been told that this won't be a problem, and indeed, several of the companies I depend on for Groups.io, including our hosting company, have not put up their GDPR mandated materials yet.
More when I know.