I work for a vendor of an operating system that has PSE54 certified conformance to the latest POSIX 1003.1 2018 Edition Specification. Our customers demand this certification as part of their ISO 26262 and IEC 61508 safety story.
POSIX certification is real and current and there are certified conformant systems out there.
Do note that the last certified Unix specification was UNIX V7. Unix is not POSIX. POSIX is not Unix.
Interesting, I wasn't aware of that.
Although, I don't see anything certified to 2018 in the POSIX certification register: http://get.posixcertified.ieee.org/register.html
However, I'm not aware of a certification program for the POSIX 2017. As far as I am aware, UNIX V7 is still the latest certification program.