It sounds like it. The methods left ‘on’ StyleContext that are not now deprecated are not really part of StyleContext at all, and in particular are not instance methods. My guess is the Vala bindings, having put them ‘in’ StyleContext as static methods, are hitting the warning about the entire class being deprecated. I don’t know if they have a way to avoid that; hopefully someone with more introspection knowlege can comment.
See also: link