Is this the case? If so, please log out and log in again to assure that there are no stale processes in your running desktop. If you still get the notifications, reboot. If you still get the notification also after this, it seems that they do not come from system-config-printer, but perhaps from Mate's desktop.
First, run the following commands to check whether your system- config- printer got completely upgraded:
dpkg -l | grep 1.5.7+2016 config- printer
dpkg -l | grep 1.5.9+2017
dpkg -l | grep system-
Is this the case? If so, please log out and log in again to assure that there are no stale processes in your running desktop. If you still get the notifications, reboot. If you still get the notification also after this, it seems that they do not come from system- config- printer, but perhaps from Mate's desktop.