Right now there is no easy way to change monitor brightness in some configurations. While power plasmoid sometimes provides a brightness slider (happens in case of laptops), in other cases it doesn't for no clear reason. Having a brightness UI right in System Settings > Display and Monitor would be helpful. For example I can change brightness like this for my monitor (at least with Kwin/X11, not sure how to do that with KWin/Wayland): xrandr --output <output> --brightness <fraction> So it's possible, but there is no UI in KDE for it in any settings.
xrandr most likely only changes the output ramps, so the lower you set your brightness, the less color steps you will see. Changing the brightness would best be done via the DDC protocol.
I also would like this feature, but I also suggest that this component be rolled into Night Colors. I might also suggest being able to set both a day brightness/temperature and a night brightness/temperature separately.
@Shmerl additionally there is brightness in "Settings -> Power Management -> Energy saving". Which is not very logical, discoverable place. It is if someone decided that the brightness of a monitor related more to a battery life, then to the monitor. That menu seems to share code with "battery" system tray. https://github.com/NixOS/nixpkgs/issues/69150 because they become not present at the same time.
Also by that NixOS bugreport can be said that we diagnosed that this issue and: https://bugs.kde.org/show_bug.cgi?id=410044 https://bugs.kde.org/show_bug.cgi?id=410957 Is the same issue KDE issue. NixOS has a lot of developers sitting right on its `master` branch, so they also hit the same bug in latest Plasma release.
As it is said - it is probably related to categorization of a power profile. Also problem was observed not only on desktop, but also on a notebooks also (Lenovo Yoga 2 for example) (maybe with monitors connected).