It's not the plan though. You can still inject global styles into libadwaita apps. It continues to be as bad an idea as it was before. I'm happy to provide details here.
So a GNOME user with the POP_OS! theme will see those styling elements when opening an app that's using libadwaita, without any special modification to the app or compile process?
Edit: I kind of alluded to this before but I want to make it clear, if you are trying to make it so the user can safely load themes without "special modifications to the app or compile process", then CSS is probably the wrong choice. It is not suited to do that, CSS is only safe to use when modifying the app itself, or if you are shipping a theme as a build-time dependency in the form of a library (i.e. libadwaita). So that's kind of why your question doesn't really follow to me.