> This has basically never been supported because x treats all screens as one big screen unless you run multiple X screens which disallows moving Windows between screens which is a pretty big barrier to normal usage

Hmm.

I'm not sure what windowing system you've been using over the past ~thirty years, but you seem to be unaware of both Xinerama (released in the late 1990s) and XRandR (version 1.2 [0] of which was released in like 2006). Maybe you've been using an X11 implementation provided by some proprietary *NIX for all of these years, but whatever you've been using, it has certainly been neither XFree86 or xorg.

> The only exception is that monitors that support variable refresh rates may be able to offer this feature in multiple monitor configuration subject to software and hardware options.

1) Monitors have been able to support multiple refresh rates for ages. This is a big part of why EDID and friends exists.

2) I note that VESA standardized Adaptive Sync in ~2009, and that VRR-supporting monitors were extremely uncommon in the consumer space until the introduction of GSync and the addition of Adaptive Sync to the DisplayPort standard... which both happened in ~2014. Add to that the fact that my secondary monitor does not support VRR, and it becomes very clear that VRR is not a prereq for driving multiple monitors at different refresh rates.

[0] IIRC, 1.2 is the version that gave it feature parity with Xinerama