Apparently, there’s a macOS bug that if your Mac has been connected to a Thunderbolt Display for a long time, the brightness of the display can no longer be adjusted (neither by pressing the according F1/F2 keys on your keyboard nor by using the brightness slider in the display settings of the system preferences).
The simple yet surprising fix is:
- Unplug your Thunderbolt Display’s Thunderbolt cable, and plug it in again
- The display’s brightness will be adjustable again
(credits: https://forums.macrumors.com/threads/thunderbolt-brightness-control-settings-gone.1254406/)