NIS+ got removed from Solaris back at the end of 2009. So it got removed from OpenSolaris and thus illumos, as that was before the fork.
This was announced long before. As a heavy NIS+ user at the time, we were somewhat dismayed to read the announcement. Sun said it would be no problem, LDAP was a superior replacement. We actually tried LDAP, and to call it an unmitigated disaster would be far too generous. Apart from the excessive hardware requirements, endless bugs, woeful performance, and lack of basic functionality, there was an almost total lack of administrative tooling.
My understanding is that NIS itself is also slated for the chop in Solaris 11 at some point.
In theory, you could reinstate NIS+ in illumos - we have the whole commit history so reverting 6874309 wouldn't be so bad. Whether you could get it to work properly is another matter.
