Plenty of older window managers don't support it correctly or use non-standard properties. Plenty of newer window managers have non-standard internal properties

Plenty of older window managers don't support it correctly or use non-standard properties. Plenty of newer window managers have non-standard internal properties for unstable features too. I would not say that Sway is doing anything special here, what they have done is exactly what I was saying earlier: placed the unstable private APIs in the Wayland protocol, with all the problems that entails. Applications should probably not be consuming those APIs directly, they will likely want to go through an abstraction layer.