It's as good as any other standard: as a baseline of agreed-upon "correct" behavior for interoperability. Anything you add from there is gravy. It's more for implementers than users, e.g. for someone writing a new shell rather than writing shell scripts. Having the standard handy is also pretty useful when writing foreign interfaces, like the posix modules in python and perl.
As for the current state of POSIX, well, you're looking at it. Might find a blog or two of someone on the POSIX committees, but the organizations aren't the kind that keep blogs. Probably best to just dive into the Wikipedia article on POSIX and start following the references on the bottom. You'll probably want to look into SUS, the Single Unix Specification, as well: it's identical to POSIX (plus curses for some reason) but it's the label that OS vendors may use rather than POSIX. macOS and some Linux distributions claim to be fully SUS-compliant; Linux as a whole does not, because its official scope is limited to the kernel which only implements a subset of POSIX.
Fun fact: the name "POSIX" was coined by Richard Stallman.