It was removed because keeping it would have resulted in too much other stuff having to be removed.
Horizon: Zero Dawn was an incredible game, an open world action RPG that perfected the template for the genre, all while looking stunning and delivering a compelling and intriguing story. It was also a rarity upon release, because it was a single player only game, with no multiplayer elements to it whatsoever.
However, during development, it turns out that it had a co-op mode planned- what’s more, this mode was apparently fully functioning too. Why, then, was it culled? It was apparently removed because Guerrilla Games determined that trying to accommodate the feature would result in too many other things being cut from the game- things they were not comfortable removing. And so, the decision was made to remove co-op instead, and make the game fully as they wanted to make it.
It was a great decision in the end, and I think the game as it exists right now would have been poorer if anything had been removed from it in a misguided attempt to incorporate co-op. Horizon: Zero Dawn is out now on PS4 exclusively.