Too long, didn't read: Right now Catalina isn't ready for prime time. You can try Catalina with Mail Archiver 5.1 beta 3, but be prepared for a rougher ride.
Security in Catalina
Whatever you read about Catalina it concentrates on the pretty stuff like no more iTunes, which has had the most confusing interface ever. What hasn't been mentioned in detail are the security changes. ALL file access is now "secured". Your applications are also more "secure". You will see more security dialogs than ever.
Bugs are part of every software and beta software has more bugs. I'm used to installing beta software of macOS - most things usually work fine, I report some bugs, a few of them are even fixed. But I have never seen a macOS which I had to reinstall after completely foobaring it.
Mail Archiver and Catalina
The deep changes in Catalina also affected Mail Archiver. Loading the preferences didn't work in all cases, the installer crashed and so on. Everything should be fixed now. You will still get "unidentified developer". But you might get this for all newer software because Apple still hasn't decided what they want to do there.
If you plan on updating to Catalina when it is available you MUST update to Mail Archiver 5.1.
As usual you can download Mail Archiver from the Moth Software website or from within the application.