This is according to a report by DualShockers (via Destructoid) where the feature was originally revealed. Apparently Ubisoft had been planning multiplayer for the game but decided at the end to not include it in the game. However the feature, or at least remnants of it, revealed itself due to a bug that occurred when a second controller was plugged into the console.
This seemed to suggest that the multiplayer feature planned would have most likely been local and some kind of co-op feature instead of online. Ubisoft only introduced multiplayer in Assassin’s Creed Brotherhood where it was of the competitive kind of multiplayer, although in Unity the developer decided to go with a 4-player co-op mode instead.
However like we said, ultimately Ubisoft decided that maybe multiplayer wasn’t the way and removed it in Syndicate. It is possible that maybe in the future they could come up with a new multiplayer mode for future Assassin’s Creed titles, but what say you? Is multiplayer an important aspect of Assassin’s Creed to you?