KDE Plasma Applet for display brightness adjustment

Setup

git clone https://github.com/JonMagon/plasma-screendimmer.git
kpackagetool6 --install package

Right-click the task bar, then click Add widgets, search for Display Brightness and add to the panel. You can move it if necessary.

Alternatively, right click on the system tray, open settings, in entries list search for Display Brightness and enable it.