Michael Tsai is the developer of Eagle Filer - a sort of competitor to Mail Archiver -, SpamSieve and DropDMG. Yesterday he warned about data loss in Mail after updating to Catalina (see his blog).
Unfortunately, his information is a bit thin on the details. A quote from his blog:
Updating Mail’s data store from Mojave to Catalina sometimes says that it succeeded, but large numbers of messages turn out to be missing or incomplete.
- Moving messages between mailboxes, both via drag-and-drop and AppleScript, can result in a blank message (only headers) on the Mac. If the message was moved to a server mailbox, other devices see the message as deleted. And eventually this syncs back to the first Mac, where the message disappears as well.
Normally, if you lose any data you notice that immediately. Here it's very likely that the problem may go unnoticed for a while. The email loss also is going to be propagated from the Catalina computer to your other devices.
A backup is pretty useless. You can't restore the data because you already got new emails which would be overwritten in the restore. An export to mbox and import of the mailboxes with "Import mailboxes" also isn't a good idea. You would end up with a lovely mess of duplicate mailboxes and emails. And you can't be sure that the problem won't occur again.
Right now it's not clear how to find out immediately if you are affected or not. I have been running Catalina since day 2 or 3 and haven't had any data loss at all.
If you have updated to Catalina then check your emails in Mail. Is the size of your Mail folder in user/Library/Mail off? Do you see blank emails when moving them? If you plan to update to Catalina make sure everything in Mail is archived to Mail Archiver. And don't use iCloud as backup. One Mail Archiver user had a lot of fun when he updated to Catalina because of that.