You have to wonder how meaningful this news is in 2024, but macOS 15.0 Sequoia running on either Apple Silicon or Intel processors is now UNIX 03-certified.
The UNIX 03 Product Standard is the mark for systems conforming to Version 3 of the Single UNIX Specification. It is a significantly enhanced version of the UNIX 98 Product Standard. The mandatory enhancements include alignment with ISO/IEC 9989:1999 C Programming Language, IEEE Std 1003.1-2001 and ISO/IEC 9945:2002. This Product Standard includes the following mandatory Product Standards: Internationalized System Calls and Libraries Extended V3,Commands and Utilities V4, C Language V2, and Internationalized Terminal Interfaces.
↫ UNIX 03 page
The questionable usefulness of this news stems from a variety of factors. The UNIX 03 specification hails from the before time of 2002, when UNIX-proper still had some footholds in the market and being a UNIX meant something to the industry. These days, Linux has pretty much taken over the traditional UNIX market, and UNIX certification seems to have all but lost its value. Only one operating system can boast to conform to the latest UNIX specification – AIX is UNIX V7 and 03-certified – while macOS and HP-UX are only UNIX 03-certified. OpenWare, UnixWare, and z/OS only conform to even older standards.
On top of all this, it seems being UNIX-certified by The Open Group feels a lot like a pay-to-play scheme, making it unlikely that community efforts like, say, FreeBSD, Debian, or similarly popular server operating systems could ever achieve UNIX-certification even if they wanted to. This makes the whole UNIX-certification world feel more like the dying vestiges of a job security program than something meaningful for an operating system to aspire to.
In any even, you can now write a program that compiles and runs on all two UNIX 03-certified operating systems, as long as it only uses POSIX APIs.