Yes, it was presumed that The Doctor sapped Missy’s regenerative abilities during “Extremis,” so one has to wonder if or how this could be possible.
As with any pivotal moment in Doctor Who, there’s another facet that needs to be considered: is Missy’s heroism coming out for the right reasons?
But there is always the chance that this process didn’t work, or that something cancelled out the regeneration -sapping, which would allow Missy to revamp herself through an extreme makeover.
Considering we saw Simm’s Master rise again, introduced with the line, “Give us a kiss, then,” one has to wonder if she’s playing the hero with expert precision specifically so she can regenerate into a more familiar and evil form.