I chimed in on the earlier thread, about having done some stuff to manage the depth buffer and add z-coordinates to polygons. I also mentioned setting it aside for a while to tackle a Vulkan backend and tease out any portability concerns. (There were a few, but nothing terrible.)
The backend is basically at parity with the current OpenGL one. When it got there, I took a second sweep through the original project: the results are now posted as a “Custom objects” series of PRs: #1-4, 5 being something new. (Vulkan’s there as well. Also, I have a consolidated branch here.)
There have been some recent discussions about starting to integrate this, though I don’t know how soon that would extend to the 3D-relevant bits, PRs #3 and 4.
Also, for anybody “looking to adopt”: I’m NOT describing a competing engine. 
Rather it’s a way to expose real hardware support through plugins; then and now, it’s relevant in that depth buffer access is one such possibility.