Since Plasma 5.10 it supports the "default" action on a notification which is invoked when clicking the notification popup itself.
It's also used by Gnome [1] and Unity, which don't have dedicated action buttons to begin with.
The option to show a visible "Open" button (shortened from "Show mail") is still avilable but I didn't even knew it existed and clicking the notification popup itself is a natural thing to do.