There is currently no hotkey to close all active notifications at once.

Notifications without a timeout (e.g. notify-send -u critical) must be dismissed manually one by one with the mouse, which is inconvenient.

A workaround using a script is possible, but it is fragile and not user-friendly:
for i in $(seq $(notify-send -p " " -t 1) -1 0); do
qdbus org.kde.plasmashell /org/freedesktop/Notifications org.freedesktop.Notifications.CloseNotification $i
done

Please add a hotkey that dismisses all currently visible notifications in a single action.