I appreciate you bringing this to our attention, but (as shadow upstream maintainer) I'm going to join John in saying this should be wontfix.
Now if you want to change the subject to also making /etc/passwd 600, then as Alexander points out that may be doable and have merit. But just hiding the backup file doesn't make sense, and as it would require extra code in the already fiddly backup code in shadow, there is regression concern.
I appreciate you bringing this to our attention, but (as shadow upstream maintainer) I'm going to join John in saying this should be wontfix.
Now if you want to change the subject to also making /etc/passwd 600, then as Alexander points out that may be doable and have merit. But just hiding the backup file doesn't make sense, and as it would require extra code in the already fiddly backup code in shadow, there is regression concern.