The Audacious audio player had been ported to GTK3 and released, but now it seems they are going back to their GTK2 roots while the Qt port of the audio player is still being developed. John Lindgren of Audacious wrote, "As anyone using Git master has noticed (or will notice soon), the GTK+ interfaces are now using GTK2 again rather than GTK3. The change was made because certain design choices in recent releases of GTK3 have made it increasingly difficult to write a 'traditional' PC / desktop program using the toolkit. The long-term goal is still to switch to Qt; however, the GTK+ interfaces need to remain stable in the meantime, and going back to GTK2 appears to be the only way to achieve this."
Audacious 3.5.x is still using GTK3 while Audacious 3.6 will be GTK2 with the possibility of a separate spin that's built for GTK3. Among the upstream GNOME design choices that angered Audacious developers using GTK3 were the client side window decorations work (for Wayland, etc), message windows being similar to Android, monochrome icons, and other factors. Again, the GTK2 revert is just a stop-gap measure until the Qt support is ready for this open-source multimedia software.