And those scripts should check they are indeed calling the gnu coreutils version before they proceed.
in the real world this would not work; it's putting the cart in front of the horse.
I would place the blame for that on the GNU coreutils project. At best, it’s feature creep. At worst, it’s embrace and extend on the POSIX standard.
POSIX doesn't forbid new flags or options. It's up to the author to read the spec and test his portability, or else willingly rely on certain distros. Some GNU tools have had strict modes as a courtesy (they used to jokingly call it POSIX_ME_HARDER).
Would they work on BSD/MacOS? It’s not just gnu vs rust out there.