A work-in-progress patch series was posted over the weekend for adding variable refresh rate support into Mutter for X.Org and Wayland. This includes checking for VRR support from connected monitors using the DRM properties, support for activating VRR, and the ability to toggle the VRR support via a DBus API. The VRR support isn’t advertised to Wayland clients at the moment for the lack of an upstream Wayland protocol around VRR.
I can’t wait for Mutter and Kwin to adopt and integrate support for variable refresh rates, so seeing these first patches is good news.