This is a new Linux explicit synchronization protocol based on DRM synchronization objects 1.
Since nvidia drivers do not properly implement implicit sync, this protocol not existing is the root cause of flickering with nvidia graphics on Wayland. This MR being merged means that Wayland might finally be usable with nvidia graphics with the next driver release.
The Nvidia driver on Wayland has been decent for a couple of years and stabilized a lot over the past ~6 months. The flickering issue was specific to XWayland. Normal Wayland apps don't have flickering problems (not quite sure why tbh), but XWayland apps would often rapidly flicker between 2 frames since it only supported implicit sync, which confused the Nvidia driver, which only supports explicit sync. Now with a Wayland protocol for explicit sync, XWayland can be updated to support it and resolve the flickering there.
Nouveau should have already been fine, this should fix the proprietary driver's issues. AFAIK this is a core issue of the proprietary driver, so should affect all cards.
I tried Wayland on my 16xx series GPU, Electron apps were only annoying, but games were unplayable. The desktop itself and Wayland native apps worked fine, though.
In addition to what others said a recent Nvidia driver update also added a workaround to reduce out of order frames without explicit sync. Ime it just made it so that resizing a flickering window makes it stop.
It's not really card related, but rather it just comes up sometimes in niche circumstances. I only had this on my second monitor and then it went away with an nvidia driver update. (since then i moved to amd)
I only have experience with my current GPU (3070 ti) and only in Hyprland, but the only flickering I have is in steam windows, everything else works flawlessly on 535 driver. Still excited that it may be fixed soon!