I've encountered a problem when upgrading my late 2013 MacBook from OS X 10.9 Mavericks to macOS 10.13 High Sierra (now running Mojave): brightness controls appear to not affect anything (only the pop-up shows, that's it), it is stuck at maximum. This problem is not present on Mavericks.
The interesting thing is that the
brightness terminal utility, available from Homebrew, works just fine, and currently that is my ghetto solution.
My suggestion is that in the process of upgrading connection of keypress and brightness adjustment got lost or flawed, and I don't know how to restore it. Nor do I know how to bodge a script to intercept signals from keyboard and adjust brightness using the utility mentioned above.
Resetting SMC as well as NVRAM (suggestions from Apple forums) didn't solve the problem.