The full quote goes like this:
The -h, -i, -n and -v options are non-standard and their use in scripts
is not recommended. They are provided solely for compatibility with
other ln implementations.
I've looked at a number of versions of the ln man page, and I noticed that none of these options appear in the POSIX spec for ln or in older versions of UNIX. Early versions had no options at all. The -s option was added when symbolic links were invented, and -f was a standard option added to a number of commands to handle a very common use case. That was it for mainstream Unix, where people preferred minimalistic toolsets.
I'm guessing, but it seems likely to me that most of the options used in popular implementations of ln were invented by Project GNU. Project GNU ("GNU's Not Unix!") started out as a volunteer effort to create a free clone of Unix. As you can see, Project GNU loves to hack out clever little features, so it makes sense that all these new options were their idea.
Project GNU never really produced a working OS (they're still working on it!), but their libraries and utilities became standard features of Linux distributions. (Which is why GNU diehards insist on referring to "GNU/Linux".) Linux features that became popular tended to be copied over to various implementations of Unix. Some (but not all) of GNU's additions to ln seem to have appeared in mainstream Unix implementations, such as Solaris and BSD. I suspect that BSD people felt that these four options were forced on them — hence the curmudgeonly deprecation in the man page.
OS X utilities are based on those for BSD, so the deprecation also appears there. I haven't found it on any system not based on BSD.