Solution Found:   My searches were focused on the immediate problem, but my colleague remarked how there were a couple BIOS updates that wouldn't install. He looked up a solution to that, which also solved the On-screen display. -In Device manager go to System Devices tab -Uninstall all Microsoft Windows Management Interface for ACPI (there's ~10) -As well as Microsoft System Management BIOS Driver. -reboot -Then Dell command update Unfortunately I do not recall which BIOS update version it was, but this oughta do the trick.  This solved the issue on both Windows 10 and 11 versions of the Dell optiplex 7400. 
Hello,    On a Dell Optiplex 7400 AIO that was upgraded by us to Win11, we are unable to figure out why the display brightness cannot be changed. The slider on Windows settings does nothing, and Dell On-screen Display (OSD) doesn't seem to function on it.    List of what I've attempted:   -updated drivers and bios -updated windows 11 to latest -attempted to install Dell OSD  -verified Windows power settings Any help would be much appreciated
optiplex 7400 display brightness unable to change