If you subtract GNOME from the set then things become a lot more sane. "Compositor-specific extensions" are really "everyone besides GNOME extensions." The system tray extension isn't KDE-specific. Sure, window positions might not be available at all (because they don't make sense for a TWM), or a user might not have a system tray bar (or you might be on GNOME). However, if they did have a system tray it would be the StatusNotifierItem protocol. Ideally, these should be handled like other platform features like accelerometers etc.. That may not be possible, either way a lot of them can safely noop.

> [Article] For Avalonia, this means "Wayland support" isn't one implementation, it's potentially dozens. We're not just writing a Wayland backend; we're writing a GNOME-Wayland backend, a KDE-Wayland backend, a Sway-Wayland backend,

If you're making per-WM backends then you've fundamentally misunderstood how extensions are supposed to work. Other Wayland client libraries do not have a independent backends for KDE, Sway, and GNOME. Maybe quirks would be needed because you're attempting to support an existing UI library - but those should be few and far between.

IIRC Avalonia supports Vulkan as a rendering backend? Wayland protocols are the same line of thinking as Vulkan extensions.

wlroot and smithay are good examples of what extensions are used in the real world.