I was in the mindset of classic RPG. And as in all (or most) RPGs, everything is about simulation.
Of course, if you have a serial killer story where you have a chance to save one of the potential victims, the implication would be massive. Or a game about robbing a bank and killing the hostage. Anything depends on the author, as I said it. If it’s not the focus, it’s usually not get added, else it detracts from the core experience.
I’m supportive of the idea to have consequences for either sparing or killing an enemy but, my biggest concern is would the consequences be executed well and have a ripple effect on the story overall. This isn’t something you’d really have to worry about if you spared or killed a major antagonist, but something I feel like could be easily glossed over if you killed a grunt/red shirt that is nameless and you only find out their story meet their relatives who are possibly seeking revenge 3-6 months later. I feel like the red shirt would need to be developed enough so the player at least understands who this person is, that way it actually matters and isn’t some random event that came out of nowhere.