The current narrative for the fighter and rogue can allow for it. We also have added things that would explicitly allow for it. They are just less-defined. Something you keep trying to claim means they cannot work, despite the less-defined sorcerer absolutely working.
Tashas.
And while I bet I know what you are going to latch onto, I'm going to give you this first.
"How did you discover this grim power? Did you sleep in a graveyard and awaken to your new abilities? Or did you cultivate them in a temple or thieves’ guild dedicated to a deity of death?"
One of the official origins of their powers is sleeping in a graveyard. That's it. Fall asleep in a graveyard. How many fighters sleep on battlefields? How many fighters sleep near sites of violence and bloodshed. If all it takes for magic soul-stealing powers is to sleep in a graveyard, then how hard is this really?
But, here's the full description
"Many rogues walk a fine line between life and death, risking their own lives and taking the lives of others. While adventuring on that line, some rogues discover a mystical connection to death itself. These rogues take knowledge from the dead and become immersed in negative energy, eventually becoming like ghosts. Thieves’ guilds value them as highly effective information gatherers and spies."
And you are going to say "AHA! A MYSTICAL connection, that's the difference, that's what you need in the rogue or the fighter to allow this!"
But, again, just sleeping in a graveyard is enough to forge this connection. The fine line between life and death, risking their lives and taking the lives of others... that's literally the same thing referenced with the fighter's "And they are well acquainted with death, both meting it out and staring it defiantly in the face." But you are demanding the word mystical, or eldritch, or magical, or some other single word. The fighter can't be "well acquainted" they have to be "mystically connected" and then you will be satisfied. Even though, frankly, death being a mystical force, being "well acquainted" with that mystical force would be the exact same thing.
Again. It isn't that this stuff doesn't exist. You just don't like the way it exists, and therefore are demanding it be changed.