On previous Plasma I was able to slide the screen backlight slider all the way to zero - screen backlight off. Now despite sliding the slider all the way to zero the screen only gets dark but backlight is still on.
Personally i think that ability to lower screen brightness to backlight off was a bug) Cause if you lower screen brightness to backlight off with mouse and move mouse cursor away from 'Battery and Brightness' widget - desktop becomes unusable
This is intentional to keep you from accidentally making your system inaccessible. If you want to turn off the screen, use the button to do that, brightness zero is not turning off.