I was the engineer responsible for harmonizing the 'mount' command between SunOS (aka BSD 4.2+) and SysV R3.2. The way that worked usually is that the command would get options from both sides of the tree and we would argue about the ones that overlapped, but in the case of mount the AT&T System engineer was totally appalled that mount had a -all option to mount everything in the /etc/fstab file. AT&T was pushing a new script named 'mountall' what would mount all file systems. They also hated multi-letter options like '-o rw,noatime'. Their argument was that there wasn't a subroutine in the C library to parse multi-letter options like we had for the mount command.
So I invented getsubopt which, with Bill Shannon's sign off, we made a 'standard part of SunOS'. And then we added it as something to be included in the consolidated lib C.
Going through that exercise of trying to harmonize System V and SunOS into the one true UNIX was the weirdest thing. AT&T had 'system engineers' who never wrote code, and Sun just had engineers who actually wrote code. Some of the technical disagreements were settled at the 'VP' level (Eric Schmidt at the time) where Eric and his AT&T counter part would 'trade' concessions that had escalated to that level. Like I said, very weird.
So the history of these things remains murky. Part of that is it's difficult to find old ("superseded") versions of the XCU / XOpen specs, let alone trace the new things each revision introduced to the sometimes-now-extinct Unix variants they first appeared in.
I do love the `unix-history-repo` and applaud the ongoing work there!
[1] https://twitter.com/alangrow/status/966081208487575552
[2] https://github.com/NetBSD/src/commit/54af00c6ee446afe395f319...
http://pubs.opengroup.org/onlinepubs/7908799/xcu/unlink.html
I couldn't get my hands on SUSv1/XCU4. It's supposedly available here:
https://publications.opengroup.org/c43x
But you have to log in to download, and I gave up upon seeing account creation form with 16 required fields.
"Open" in "The Open Group" is a bad joke.
FWIW, all OpenBSD man pages have a history section at the bottom that notes when the command first appeared.
The manpage you pointed to is about the "unlink" system call.
Personally, I tend to focus on GNU coreutils on the gnu/linux side.(https://www.gnu.org/software/coreutils/manual/coreutils.html)
I can't find my list of non-gnu linux kernel utilities at the moment. I think it's in the older kernel documentation.