GTK+ theming engines no longer supported in 3.16+?

From what I've heard over at https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=822976, GTK+ 3.16 and above drop support for theme engines entirely, rendering projects like Unico unusable. However, I'm not sure how to confirm this, or how to tell whether themes are already using pure CSS or a combination of CSS and theme engine elements.

In Ubuntu, there still seem to be some reverse dependencies for gtk3-engines-unico, even though GTK 3.18 is available in xenial:

gl@millennium:~$ reverse-depends gtk3-engines-unico -r xenial
Reverse-Depends
===============
* lubuntu-artwork-11-10
* lubuntu-artwork-12-04
* lubuntu-artwork-12-10
* lubuntu-artwork-13-04
* lubuntu-artwork-13-10
* lubuntu-artwork-14-04
* lubuntu-artwork-14-10
* lubuntu-artwork-15-04
* lubuntu-artwork-15-10
* lubuntu-artwork-16-04
* mythbuntu-default-settings

Could someone please clarify the status of this package?