After eight games that feature local Campaign co-op the mode has apparently been scrapped for Halo 5.
http://uk.ign.com/articles/2015/06/09/xbox-live-gold-not-needed-for-halo-5-online-co-op
However, Halo 5, unlike previous series entries, won't support split-screen co-op.
http://uk.ign.com/articles/2015/06/09/xbox-live-gold-not-needed-for-halo-5-online-co-op