It is official. Nice catch. Thanks. I'll look at adding that to the website.
$ grep EXPIRATION_DATE /usr/ports/Mk/*
/usr/ports/Mk/bsd.port.mk:# EXPIRATION_DATE
/usr/ports/Mk/bsd.port.mk:.if defined(EXPIRATION_DATE)
/usr/ports/Mk/bsd.port.mk: @${ECHO_MSG} "It is scheduled to be removed on or after ${EXPIRATION_DATE}."
$