On the one hand, Adobe's stated min VRAM for PPro v13 is 4GB. On the other hand, cards with less than that are listed as CUDA compatible.

I gave PPro 2019 (v13) a very quick test run today on a mid-2014 MBP with a 2GB VRAM 750M card. CUDA and Metal were clearly working. I didn't try OpenCL.

The OS was High Sierra. Nvidia says that Apple isn't currently supporting CUDA in Mojave:

https://devtalk.nvidia.com/default/topic/1042279/cuda-10-and-macos-10-14/

perhaps much the same as OpenCL is being deprecated?

http://www.dvinfo.net/forum/adobe-creative-suite/535936-bad-news-apple-eols-opengl-opencl-macos-10-14-affects-creative-cloud.html

https://theblog.adobe.com/important-information-on-gpu-acceleration-with-cuda-and-apple-metal/

Here's to hoping that Metal is really ready for prime time!