It is optional and not up to date with the latest specification.

The problem with POSIX, like any other standard, is that not all systems implement the same POSIX.

I'm not a porter so I'm just trying to understand here, but I would suspect that having POSIX, albeit quirky, would automatically make it easier to port than Windows which has no POSIX. Like porting a web app to IE6 is easier than porting it to a Node CLI app. Does that analogy not work?

Windows has a POSIX implementation.

https://en.wikipedia.org/wiki/Interix

The problem is that each POSIX compatible system tends to have different levels of support for the various POSIX standard versions.

Additionally there are APIs which have undefined edge cases or different limits (e.g. amount of open file handles).

So even if you only target POSIX systems, you have to pollute the code with #ifdef to handle such differences.

It is similar to do web development. There are standards, but each browser version is a different world.