gtk-doc 1.25 has dropped support for docbook sgml via java ( https://git.gnome.org/browse/gtk-doc/commit/?id=4a9efdc49184df82a28b38534936f8dd5c5864a9) This means openjade (and possibly app-text/docbook-sgml-dtd too) are no longer its deps.
...oops, seems I've copy-pasted a typo... Oh, not like it really matters.
The version in tree does not depend on openjade. I think leio noticed and fixed this independently.
... well, on a semi-related note, for quite awhile, the upstream version has been python based, with some functionality removed (as it's no longer used) and quite a few bugs fixed (as I've noted in one of those bugs).