Software brightness control on desktop monitors is something I've wanted for a while. Closest thing I've seen is a monitor with a built in light sensor that has the option of auto adjusting independent of the computer.
Taking my laptop as an example: Windows has two APIs for adjusting screen brightness, one of which only works for the internal display and one of which only works for external displays; and sadly the brightness keys on the laptop are uninterceptable and I have not come up with any way of linking the brightnesses either. I went hunting and settled on some old freeware called ScreenBright which I can invoke from the command line, so that now I just run `b 0` for night time and `b 40` for most of the day (and up to 70% in certain seasons—but 100% is pretty much always too bright where I use my external displays). Since I arranged that workflow, I have also written a tiny Rust program that interacts with the APIs directly which could replace it.
This doesn't seem to be true for most laptops. The ones I have used show a OS graphic when you turn the brightness down similar to the volume. Have tested this using linux on a dell xps and a macbook.
Unfortunately both still do not adjust the brightness of external monitors.
The trend for decades now has been to have buttons only, and often there's a menu you have to navigate through in order to actually get to the brightness settings. It's a real hassle on many monitors.