Libadwaita: Splitting GTK and Design Language

apps written explicitly for libadwaita will not be usable on generic GTK.

When an app targets a platform-specific library like Adwaita it explicitly forgoes supporting generic, cross-platform GTK.

Will an app dependent on libadwaita be usable on linux without gnome? Like xfce, or xmonad?

Depends what you mean by "usable".

Will it run, yes, most likely. But the UI will be full of Gnome-y things that make little sense on other DE's, among which the theme is just one. Could be other stuff, like accessibility, features etc.

As long as none of the shortcomings are deal-breakers for you I guess you can call it "usable".

At the end of the day there's always going to be DE's that expand their UI features, and apps that take advantage of those. Cosmic, Plasma, Granite etc. are all examples of such platform-specific UI libs. Even in a cross-platform library like GTK or Qt there's no guarantee that they're compatible with each other. Bottom line is, when you mix and match apps made for different toolkits there's always going to be variations.

It was nice for a while having common themes that could target both GTK and Qt and having unified-looking desktops. I guess that era is over. Back to each app having its own incompatible theming and the only common point being that they're all "light-ish" or "dark-ish".

Windows supports libadwaita

It means that on systems with apps installed written with libadwaita, will also have libadwaita installed, rather than just GTK. But those apps will look like GNOME apps, which might look out of place on e.g. a Windows or Xfce desktop.

Will an app dependent on libadwaita that be usable on linux without gnome? Like xfce, or xmonad?

of course it will, that's not the point, the point is to make apps that use libadwaita look consistent even in platforms outside of GNOME