You could use newsboat -x reload && newsboat -x print-unread as a good start to get the status of unread articles and print a notification (for example using dunst) when the number increases.

Yes, but not for a specific feed. It’s all or none. You could make individual url files for each feed and call them in sequence with cron or something, but that’d probably get unwieldly. You can also (I think…) control the format of the notification.