Legacy backup will not work in the recent versions of Cpanel (50 and later) even if we enabled the the same in backup configuration file '/etc/cpbackup.conf'. In this case we need to enable backups for each account manually via WHM front end panel (Home >> Backup >> Legacy Backup Configuration).
We can do the same via command line by editing the user configuration files.
User configuration files for all cpanel accounts are located in the folder '/var/cpanel/users'.
We need to update the line "LEGACY_BACKUP=0" to "LEGACY_BACKUP=1" in all user configuration files.
To enable Legacy backups for all upcoming cpanel accounts we need to update the same in cpanel user configuration template.
For this set up, modify the line "BACKUPENABLE: 'no'" to "BACKUPENABLE: 'yes'" in the template file '/var/cpanel/backups/config'.
Please note that Cpanel planning to remove the Legacy Backup Configuration interface from WHM in a future release. If so we can use the above steps for setting Legacy backup via ssh.