With the latest Skype Preview build version 18.104.22.168, Microsoft is bringing back online notifications, an old Skype feature. With this update, when someone from your contact list comes online, you will get a notification from Skype. Yes, you will have full control over this feature. You can opt in, or out, of this new notification feature. To do so, go to Settings –> Notifications –> Contact comes online notification.
Try out this new feature and pass on the feedback to Skype team to further improve the experience.