After T108190, there are two categories for notifications, Alerts, and Messages.
However, Minerva Neue has only one button for notifications for some reason. Behind the scene, Minerva just places a <a href="…/Special:Mytalk…"> element and runs SkinMinervaReplaceNotificationsBadgeHook, then Echo takes the hook and provides a notification button.
This should be possible for other skins. It would be a simple renaming of SkinMinervaReplaceNotificationsBadgeHook, or be a more generic means in the perspective of MediaWiki-Core-Skin-Architecture.