alt Hacker NewsXorg does per-monitor DPI and per-monitor refresh rate. Debian probably never shipped a version that does, but it works fine on Gentoo Linux.
I've tested per-monitor DPI before, and [0] mentions one way to do it. I tested per-monitor refresh just now. Using the xrandr CLI to set the refresh rate to 24.0 on my primary monitor and 60.0 on my secondary results in "cinematic" visuals on the primary monitor and "soap opera" visuals on the secondary.
I'm currently using Windowmaker, but I see no reason why this wouldn't work with KDE.
Maybe it's possible now. It wasn't back when I last used X. Now that Wayland is the default on most distros and works on nvidia now I don't see any reason to go back.