We do know a little. Soulkiller is some kind of program Arasaka developed. It's part of their Secure Your Soul things that rich people are buying. And it is obviously used to kill and store the minds of Arasaka's enemies. I just always thought only old man Arasaka's Biochip 2.0 was capable of saving an 'intact soul' or something along those lines. Johnny was the prototype project they used on dead bodies to perfect the old man's own Biochip, he was shown as being old - a WW2 vet even.
But apparently engrams are already perfect. Don't know why it's even mentioned the soulkiller program was upgraded when it could apparently store perfect copies of the old man and Johnny (decades ago in his case). I had assumed that Saburo would have his mind stored on the chip when I first played Act I, didn't even think about how and when Keanu would be introduced.
It's just too vague and needed more options. You're a Cyberpunk setting with AIs roaming the net. Why not have more with godlike AIs (like Alt), merging V and Johnny? Pacifica and the Blackwall was explored once and then left on the sidelines. Instead we're playing blue-pill/red-pill all game.
Technically since 2077 is using the same lore as 2020 we also know that Alt made a soulkiller digital haven for soulkiller victims which was is the net but people don't seem to be basing themselves off of that even though lore is the same.
Soulkiller was devised to be employee-only and was used to achieve immortality, it was never supposed to go on market and wasn't supposed to "kill" the soul, it was supposed to take over a dead body and repair it, wiping the previous host's emotions etc. away and storing in the guests' then it was used on Johnny as a weapon.
The wiki also says Hanako is one of the few people that understood Alt's work on soulkiller but then again, an engram, is it really you? Are you a soulless AI? A soulless being, are you yourself or someone pretending to be you and being really good at it? Wiki of 2020 claims it's the only way to achieve immortality and was meant to be that way for high end corpo people only within Arasaka.
I think it's just more of a moral debate sure, you're a construct but you're afraid of death, emotions make people human, no? I think this is the point where most people are split as to whether this ending is good or not.
Monks in CP77 can argue that cyberware is humanity's doom as they're body purists, others are the exact opposite however.
My take is, soulkiller is just a name, it doesn't *kill* your soul, makes it feel like it in the process, it just copies your conciousness and memories which is nothing more than your brain working in the same way as it did before, laying out the same connections/feelings in your brain, the body is just a shell.