Ifdefs would be required. Or some kind of wrapper in gnulib. Or, like systemd, just non-portability. Your philosophy means stagnation forever. I reject that. Programs that use getenv today will not break. Objection to the existence of a getenv_s is absurd.

The only way we move forward is by trying new things, and we can't possibly get everyone to try a new thing all at once. If the thing ends up being useful, we standardize it.