I have an iMac/retina running High Sierra. When I bought it last year, I sadly learned Apple stopped creating Thunderbolt display and Cinema display, and instead recommended some 3rd party monitors (the two 4K Dell monitors I got among them).
Unfortunately, Apple doesn't seem to have addressed the problem of controlling their brightness through the keyboard, and that's much-needed functionality - to be able to dim them all together via the brightness key. That used to work, with my Thunderbolt display and previous iMac. So is a major problem, because adjusting display brightness in unison is expected, needed often used functionality. So it looks like we lost more than the Apple monitors/look-and-feel. We lost some essential integration.
Changing the brightness of the Dell monitors through their physical menu button on each of the two each time I change my iMac display brightness is extremely tedious and annoying. It involves several steps to reach the brightness adjustment interface.
I've looked for apps that resolve this, but there is not much out there that does it. I found one app on the Apple App store that works it by inserting an opacity changeable layer. It's sort of works but it's not implemented properly becauseit ruins screen capture - The layer they insert to darken the machines gets captured with the screen captured. I've found other flakey and unreliable apps too that work more organically, but they keep exiting so are not running when I need to change the brightness and I need to relaunch.