I am using EndeavourOS on a desktop.
In Plasma 5, there was a brightness icon, scrolling on which changed the Brightness.
Plasma 6 shows the battery icon, but I don’t have a battery. Scrolling on the icon does nothing. However, I can right click → Configure energy saving → Change brightness from there, which is inefficient.
I realized that Brightness and Color was the red lightbulb icon (it has been merged with Night Light in Plasma 6), scrolling on which does change the brightness. The battery icon was redundant.
I am using Papirus Icon Theme, and system tray icons for Plasma 6 are slightly messed up, but it has been reported upstream.