Board Thread:Versus Debates/@comment-32466165-20170722022140/@comment-25122992-20170803190839

Susanoo has always ended up in the jutsu section within the databook, and it was classified there as such (ninjutsu and doujutsu). Also, Itachi called it a technique that was awakened along with his other MS jutsu, Amaterasu and Tsukuyomi. So your assumption about Susanoo as a "living being" is highly farfetched and isn't actually true.