I would like to make a plea to add NV12 colour space.

NV12 give better performance than the traditional YUV and is recommended by all hardware vendors.
AMD, Intel and Nvidia all say this is much better then YUV.
This is a format fully specific and optimized for computer processing and especially for the video hardware blocks in the gpu.

A good quote from intel:
Intel:
Decode/encode and VPP operations use NV12 because this provides better performance than other formats such as YUV.
While it is possible to include color conversion filters it is best if pipelines can be arranged to minimize conversion steps by doing as much consecutive work in NV12 as Possible.
NV12 is designed to minimize copies arrange pipeline steps to reduce surfaces in the same location instead of copying them between cpu and gpu.

This is already the default standard in OBS, why does Vmix not have this function.
Seems like a fairly small thing to implement with great benefits.

I don't agree at all. I would much rather vMix did all its internal processing at 4:4:4 YUV like it does now, and then flip it to NV12 at the output stage for streaming/recording.