Apple Inc.: macOS version 14.0 Sonoma on Apple silicon-based Mac computers
Apple Inc.: macOS version 14.0 Sonoma on Intel-based Mac computers
IBM Corporation: z/OS 3.1 or later with: z/OS 3.1 or later Security Server and z/OS 3.1 or later C/C++ Compiler on IBM Series Processors that support z/OS Version 3.1 or later
IBM Corporation: AIX version 7, at 7.2 TL5 (or later) on systems using CHRP system architecture with POWER™ processors
IBM Corporation: AIX version 7, at either 7.1 TL5 (or later) or 7.2 TL2 (or later) on systems using CHRP system architecture with POWER™ processors
Hewlett Packard Enterprise: HP-UX 11i V3 Release B.11.31 or later on HP Integrity Servers
IBM Corporation: AIX 6 Operating System V6.1.2 with SP1 or later on Systems using CHRP system architecture with POWER™ processors and 2, 8 or 128 port async cards
IBM Corporation: AIX 5L for POWER V5.3 dated 7-2006 or later on Systems using CHRP system architecture with POWER™processors
IBM Corporation: z/OS V2R1 or later with: z/OS V2R1 or later Security Server and z/OS V2R1 or later C/C++ Compiler on IBM zSeries Processors that support z/OS Version 2 Release 1 or later
The SCO Group, Inc.: UnixWare ® 7.1.3 and later for single and multiprocessor systems based on IA-32 and compatible processors and conforming to PC architecture
The SCO Group, Inc.: SCO OpenServer Release 5 and OpenServer Release 6 on Single and Multi-processor Industry Standard Intel architecture platforms
The work is still in progress. It's also important to note, that Zephyr is intended to run on resource-constrained systems. There is not much logic in implementing fork() for example, since technically, there is only one process. So it implements a subset of the interface. Here is the link to the talk from the EOSS 2024 where Chris Friedt shares the current state of POSIX support in Zephyr: https://www.youtube.com/watch?v=nLnmXxXuS6w
What i wanted to get across is that claims about "Posix compliance" might have wide variance in meaningfulness unless backed by a certification.
Even without certification, POSIX remains a useful baseline for implementing and consuming cross-platform system services, taken with your suggested grains of salt on the consumer side for any details not specifically documented as POSIX-compliant.
UNIX certification seems mostly only relevant in terms of OS marketing, in the sense that it signifies a vendor's intention to provide a usable POSIX-ish programming environment.
As a target for cross-platform application developers, the list of certified UNIX platforms is at best weirdly specific:
macOS: supported
AIX: supported, though even IBM seems to prefer Linux for new development; IBM-supported Linux runs natively alongside AIX on the same hardware, further limiting appeal
z/OS: supported, but only a "UNIX system" in that it offers a POSIX compatibility layer on top of a system that's fundamentally non-POSIX in design (EBCDIC, expensive process creation, etc.); IBM-supported Linux runs natively alongside z/OS on the same hardware, further limiting appeal
HP-UX: supported until 2025-12-31; Itanium-only
SCO UnixWare and OpenServer: supported (?), but no substantial updates since 2018 (?)
I reckon it is because of the lawsuit they got because they used unix in marketing without actually paying for the certificate. see - https://www.quora.com/What-goes-into-making-an-OS-to-be-Unix...
From what i've seen Apple gets every Mac Os release UNIX-certified, which is somewhat cool.
But frankly, is it of any actual relevance ? Nowadays it's gnu/linux in (probably) 95-98% of the industry and the rest is likely to be shared across FreeBSD and a few other players.
But in general, I'd say any "POSIX implementation" that's not been certified should not be taken any seriously: as an example, i had a friend that was working with the Zephyr RTOS which claimed to have some kind of posix compliance... Except it wasn't full and it broke in subtle ways. Still a good RTOS (it seems, not my area of expertise) but the "posix compliat" claim should be taken with many grains of salt probably.