That still seems to be there, even for gtk4: https://docs.gtk.org/gtk4/broadway.html

(but it says, between the lines, mostly stale I guess)

As an aside: one of the very first demos of LibreOffice in a browser (from the Collabora people) was using the broadway backend. But they've moved on, using their own tiled rendering server backend, plus custom, javascript gui on the client.