Killing off the black lesbian is 100% something Steven Moffat would not do and you’ll see that on Saturday.
I don’t know if anyone’s up for some HUGE SPOILERS!!! but-
There are a couple of folks on Gallifrey Base who have seen the episode already, and although they’re obviously sworn to secrecy about what exactly happens, they’re pretty adamant that Bill’s apparent cyber-conversion doesn’t stick.
And even more promisingly, someone’s gotten a picture of Pearl Mackie on the set for the Christmas Special! (I will not link to it as I don’t want to be sued into oblivion by the BBC.) Which admittedly means very little on its own, as it could be a flashback or a dream or who knows what, but considering that a few days ago we didn’t even know if she was in it…