I'm not even sure about Mac, but in Windows, there's no distinction, because there's no "client" and "server". But from developers' perspective, it's less about CSD vs SSD, and more about being able to say, "I just want standard window decorations, please and thank you", vs not being able to.

Gnome devs' suggestion to just use Gtk3 for this is not really equivalent, because on both Windows and macOS, the library that offers the window creation API is core part of the OS - it's technically a dependency, yes, but it's the one that nobody actually cares about. On Linux, Gtk3 is a dependency that people do care about, because it's not guaranteed to be present, and requiring it often means doubling the sheer number of packages that an app like that might need.

Gnome doesn't care, because they see themselves as a platform akin to Windows and Mac - so if you intend your app to run on Gnome, then you compile it for Gnome (and then assume that Gtk3 is always there). But, of course, this attitude amounts to saying that they don't really care about compatibility with apps for "just Linux". Which is exactly why they're getting so much pushback on this.