This requires adding cron to the Full Disk Access list.
Open System Preferences : Security & Privacy : Privacy : Full Disk Access
Both rsync and cron must be included on this list. Before Catalina, listing only rsync would work. On Catalina, rsync by itself only lets you run rsync by hand. For it to work within cron, cron must also be on the list separately.
(In the Full Disk Access preference pane, hitting Command+Shift+G will allow typing /usr/sbin/ in order to then choose cron)
