Time Machine says: "Backup not encrypted"

I started a new Time Machine backup of my 2018 Mac mini which I recently upgraded to Catalina. I'm backing up two disks: the internal drive and an external drive (different from the Time Machine backup disk). Neither disk being backed up is encrypted (FileVault is off), so why does Time Machine put up this notification? No details are provided, so I don't even know to which disk the message refers.