First off, the work is not complete yet. E.g. there is no CMYK image mode. You can open a CMYK image with automatic conversion to sRGB, you can also do a soft-proof, and you can export CMYK images (JPEG, TIFF, PSD). This part is comparatively cheap to do, but it needed some infrastructure in place and, above all, it needed someone to write the code.

The previous attempt to add CMYK exporting didn't go well. The developer implemented all that in a new set of plugins instead of building the functionality into existing code. This wouldn't work. The developer apparently wasn't interested to do it right or did not have the time, he wouldn't say.

So there's the architectural aspect of this and then there's availability of a contributor willing to hacks on this.

You can read a little more about this GSoC project here: https://www.gimp.org/news/2022/06/03/cmyk-in-gsoc-2022/