> as we know it today

An important nuance you seem to be missing is that SUSv3 is equivalent to "IEEE Std 1003.1-2001" (that is, POSIX 2001).

In practice, I've had to work around more POSIX compatibility issues in macOS than in all other actively developed (Free) Unix-likes, combined.

Mayhaps not with a `cat(1)` alone, but really they just need to expand their menagerie now.

Like macOS, my cat does not qualify for the UNIX standard out-of-the-box and I'm far too lazy to configure my cat for an OS standard that's 25 years obsolete.