DDC is how monitors send EDID information about their capabilities to the computer. DDC/CI adds the ability for the computer to send commands to the monitor for things like brightness and color settings and input selection. So Windows obviously is using plain DDC to offer you the right set of resolution choices, but none of the DDC/CI functionality appears to be exposed to end users, making it effectively unsupported and unused and untested.
I don't recall seeing anything like a monitor brightness slider in their control panel. And even if there was, it wouldn't be a reliable indicator of OS-level support for that functionality because the GPU vendor's control panel is the most likely piece of software to bypass the OS and control the display config through its own means.
