I really don't think it's useful to speculate on what is/is not cut content. We just do not know enough about how development was done to know when certain stuff was added or cut, or why. I prefer to base my assessment on what we actually got, because that's something we do understand well.
Again, the major issue I think leads to so much frustration isn't so much "Karlach gets only bad endings" (whether or not there was a cut "good ending" doesn't change this much, IMO) as "Karlach is the only origin companion with exclusively bad endings, and whose quest gives you no meaningful ways to improve the outcome."
Just contrast the amount you can do- and the crap you can go through- in any of the other characters' personal quests. You can invade Hell! You can try to out-bargain a devil! You can fight an ancient and powerful vampire! - to Karlach's quest, where you're told she's going to die, and both Karlach and the MC basically just accept it and give up on trying to fix it.
