The practice of severing the ring finger for assassin initiates was discontinued by Altair, the master of the Brotherhood, sometime after its reformation in 1192. This was referenced in the video game
Assassin's Creed II (2009), occurring decades before the events in the movie. As the movie and its sequels are original stories set within the same universe as the games, removing a finger to use the hidden blade has not been necessary for centuries by the time Aguilar joins the Brotherhood. Ubisoft and the Assassin's Creed franchise team explained that the Spanish branch of the Brotherhood to which Aguilar belongs still severs one finger per initiate to ensure the commitment and loyalty of each initiate to defend both the Brotherhood and the Creed. This is why Aguilar has the ring finger severed on only one hand despite wielding two hidden blades; the severance is no longer about function, but rather about symbolism and dedication.
