If the spec was not proprietary, Nissan would have a licensing program for the NS2/NS3 specs - similar to how Ford runs their Mercon LV licensing program, or how GM runs their Dexron VI and Dexron HP programs.
The NS2 and NS3 specs are proprietary and any aftermarket fluids are reverse-engineered examples. The additive suppliers (e.g. Afton, Lubrizol, etc) are selling additive packages (with a recommended base oil mix) to blenders. I doubt most of these blenders have the same technical expertise and resources as the additive suppliers.
Example:
https://www.aftonchemical.com/Afton.../PDS/HiTEC-4022A/HiTEC-4022A_PDS.pdf?ext=.pdf