Wayland as a protocol was designed around CSDs, protocols for SSDs came years later
That's not an argument for anything. The core protocol isn't useful on its own, you always need extensions that came later to even create a window. As another example, Wayland as a protocol was designed around shared memory buffers, protocols for hardware acceleration came later. Doesn't mean you're supposed to leave that out.
Modern apps tend to prefer CSDs anyway since it provides more flexibility, very common on MacOS and Windows
That too is not an argument for not implementing what a ton of apps need.
MacOS and Windows don't do the same sort of CSD as Gnome FYI, it's more of a hybrid approach, where parts of the decoration are rendered by the system and parts by the app.
It's difficult to coordinate things between the client and compositor.
That too isn't relevant, libdecor doesn't coordinate shit either. And if you want to (which is being looked into), you can absolutely sync things with SSD too.
The actual and only reason Gnome doesn't support SSD is that they think CSD is a "better architecture".
Why would you run it in Proton? It's a native game.