Downgrading from Little Snitch 6 to Little Snitch 5 is possible and restores your configuration to the state it was in before the upgrade, i.e. all changes in 6 are lost (except for traffic history).
You can do the downgrade by installing Little Snitch 5 from the DMG file from our web server over the existing Little Snitch 6 installation.
Remember to quit the Little Snitch 6 app if it is running, otherwise macOS cannot overwrite it in the Applications folder.