Cycles (Blender's built in path tracer) with some modifications. (Cryptomattes are now in 2.8, and the Embree patch will be integrated somewhere after.)

"Yes, cycles was used for everything, though our version of cycles was modified (with stefan's embree core, and crypto-mattes which were beyond valuable for compositing). The version of blender we used was the studios own dev version (which I believe was using blender 2.78 as it's base?)"

https://www.blendernation.com/2018/08/20/next-gen-blender-pr...