https://www.opengroup.org/openbrand/register/brand3617.htm
https://www.opengroup.org/openbrand/register/brand3622.htm
Save these links for the next time someone moans that Linux "is not a real Unix".
In order to get a Linux distro certified, you'd have to make changes which would make it less compatible with all the other Linux distros out there.
The reason why RedHat doesn't pay for UNIX certification is that their distros wouldn't be compliant. The reason why they don't make their distros compliant is that their customers would vastly prefer that RedHat use "standard Linux" tools than replace them with UNIX-compliant ones. Customers don't want a Linux distro that's subtly different/incompatible compared to what everyone expects in a Linux system. They'd rather it be not-UNIX.
Yes, you can modify a Linux distro to be UNIX. However, most Linux systems are not real UNIX - and you wouldn't want it to be real UNIX.
the GNU userland might be common for user facing systems, but it's nowhere close to standard.
If you are going there, I also consider Kubernetes optional.
When I have the option to push for specific cloud deployments, I usually push for serverless or managed containers.
You might argue that still depends on Kubernetes, for me Kubernetes just like Linux, is mostly an implementation detail, that we get to open support tickets when it doesn't go as planned.
No one needs to hurt themselves running a local Kubernetes cluster.
I'm not sure what you mean by "Unix specification". But if you mean the international standard POSIX, yes, people care. Red Hat routinely participates in POSIX spec revision.
There are a very few deviations where you have to enable "POSIXLY_CORRECT". If that's what you mean, then you can turn that on. But in every area that matters, Linux distros implement the POSIX spec by default, and you can even turn on the POSIXLY_CORRECT mode to exactly follow it. They extend beyond it, but that is allowed and expected.
The people who build the tools in Linux distros care a lot. I know the implementors of dash and GNU make routinely refer to POSIX. The Linux distros don't have to as much with POSIX because that is generally a conpleted work and it's the maintainers of the tools who must address the updates to POSIX.
You might say "their exact view of what UNIX is isn't important and POSIX is," but POSIX is not the UNIX spec. You might think the Unix spec isn't important - and it really isn't today. Linux generally targets what is important and what users care about - and that isn't the UNIX spec. It is often the same as the UNIX spec, but not always and there are deviations.