Could be wrong, but I want to say the lore went in a "Rasputin actually had full control of SIVA the whole time" direction. Because of that, Rasputin's "death" means nobody is able to control SIVA anymore. Since the Replication Chamber is gone, no new SIVA can be created (other than in Outbreak Perfected). Neomuna kinda has SIVA, but it's a modified version that the people of Neomuna have full control over. I will note, I'm calling BS on Rasputin's death. I'm sure he's dead because Bungie wanted to end that storyline, but in reality, Rasputin is an AI. He's software. Meaning, every single piece of hardware in the galaxy that he's touched potentially has fragments of him left. So, unless the witness is hunting down all RAM, CPUs and GPUs in the galaxy and hitting them with a drill, Rasputin absolutely could return. If that does happen, SIVA will probably return, but more in the same way we had the Black Armory. Could have done way more with it, but we just stuck to the guns Ada already knew. My hope is Bungie will have the creative direction to have Banshee, Ada and Rasputin work together to give us a super arsenal of weapons infused with Black Armory tech and enhanced by SIVA. Doubt it'll happen, but I can dream.