Yeah, they won't have a chance to rewrite cp and mv in Rust.
Posix is more an art form documenting the far gone past rather than a practical tool
We also test GNU coreutils on macOS. It is a bit tricky since the other maintainers and I don't own any macOS machines for testing. But we seem to do well enough with a shared system that Adélie Linux kindly allows the compile farm [1] to use, along with some additional CI machines.
Apple can't distribute them - coreutils is GPLv3, which is incompatible with Apple's immutable system volume.
https://www.gnu.org/licenses/gpl-3.0.en.html#section6
(There are other issues with GPLv3 which make it a dubious choice for companies like Apple, but this one is particularly problematic.)
But the only concrete example I can think of is that `cat -n a b`, if `a` doesn't end in a newline, produces something that is obviously not what the user wanted. GNU cat does it right.
I didn't report it to the openbsd people because I didn't want them to yell at me that `-n` is nonstandard anyway with no "right" behavior, so I should STFU. Which would be what I'd expect from having reported problems in the past.
Personally, I'd remove even the POSIX [-u] flag from all cat versions because all of them are un-UNIX-like.
Some GNU tools are particularly ridiculous with the Easter egg features I only know about because LLMs use them all the time —and I have been using Linux for 30 years.
The restarting of the counter on OpenBSD is not what concerns me. The merging of the first file's last line with the second file's first line is.
Someone could have thought it was a good idea and no one was willing to audit all of UCB's scripts to fix the "bug" even if it had been the result of a lazy implementation because ed(1) on a terminal.
So I wouldn't refer to it as "the design". It's not that the methods are unsound, it's that I don't see any method at all.
As for not wanting to break existing scripts, OpenBSD is not even remotely as concerned about backwards compatibility as Linux is. I don't mean that in a derogatory way. It's a perfectly cromulent choice.