It seems to be quite possible that they killed each other, which is illogical for The Master because after declaring that "I will never be on the Doctor's side" he kills his future self ("Don't try to regenerate, it was full power"), this after staggering into the entrance of the lift, which could get him to his TARDIS for regeneration. He has been mortally wounded by his future self. So, are they both dead, both alive or is one alive and if so which one? Missy has been 'killed' at least twice before this, once by a cyberman and once by several Daleks. But on both those occasions she vanished and was teleported away. This time she did not vanish.

We now have a Chibnall era Master. Chibnall may leave the question unresolved, thinking "It's my Master now, don't bother about the others". But by virtue of the fact that there is a new Master one of Missy and the Master (John Simm version) must have survived? Well, this need not necessarily be the case. The new Master could be a prior generation of either of the 'dead' ones. If one or both survived he could be a future regeneration of Missy or he could be a regeneration of The Master (John Simm version) but before Missy. The way that it is written none of The Masters knows much about the previous versions.

Michelle Gomez believes that whatever the outcome is it is over for her, though she would be willing to return.