If you’re using an Apple Cinema Display as a secondary monitor for your MacBook laptop computer or iMac, you may have noticed that sometimes the brightness control is missing from System Preferences > Displays.
There may be a variety of reasons for this, but one cause is easy to fix. If the USB cable from the display isn’t attached to your computer, the brightness slider will be missing. The display will continue displaying the brightness that it last had until you connect the USB and change the brightness level again.
This was tested and documented using a 2008 era 24″ Apple Cinema Display connected to an Apple iMac (21.5-inch, Late 2013). The issue likely impacts other system configurations.
Iowa City Technology Services 
Apple server told me that we cannot control Apple LED Cinema Display after macOS 10.9. Terrible😫
I had the same issue.. then i read somewhere to use a different cord/adapter to plug it into my MBP2018. once i switched adapters, it’s working…. go figure… hope this helps. (the one that works for me is Amiroki Mini-Display port to usb-c, but that’s just me).
Thanks for sharing.
Excellent tip – fixed my problem. Many thanks!
Thanks!