It's definitely *not* the case that NIS is "Linux only": it was originally called 'yellow pages' and was invented by Sun. So it shipped with Solaris, at least. It was then ported to just about every Unix out there, including Linux.
It was invented long before LDAP, but could reasonably be seen much as Microsoft's Active Directory is: a proprietary implementation of a network directory service. Obviously, NIS is no longer proprietary, but it certainly started out as one company's way of dealing with distributed, replicated network authentication and identification.
LDAP is a more modern, standards-compliant way of achieving the same thing, but it is certainly a subtler, richer way of doing it -and does include encryption baked-in. NIS+ has built-in encryption but is trickier to get working than the original NIS.
I doubt anyone would plan on implementing NIS anymore, and NIS+ would likely also not be considered.