This wasn’t true in 2003 when I started using Linux in fact the feature is so old I’m not sure exactly when it was implemented. You have always been able to have different resolutions and in fact different scaling factors. It works like this
You scale your lower DPI display or displays UP to match your highest DPI and let X scale down to the physical size. HIGHER / LOWER = SCALE FACTOR. So with 2 27" monitors where one is 4k and the other is 1080p the factor is 2, a 27" 4K with a 24" 1080p is roughly 1.75.
Configured like so everything is sharp and UI elements are the same size on every screen. If your monitors are vertically aligned you could put a window between monitors and see the damn characters lined up correctly.
If you use the soooo unfriendly Nvidia GPU you can actually configure this in its GUI for configuring your monitors. If not you can set with xrandr the argument is --scale shockingly enough
Different refresh rates also of course work but you ARE limited to the lower refresh rate. This is about the only meaningful limitation.