Persephone Hazard was very briefly mentioned in Chapter 12 of 'The Nightmare Stacks' but played no other role in the story.
Excerpt from Chapter 12 (bold emphasis by me)...
this is understood by all present. Johnny doesn’t answer to anyone except Persephone.
One page further in the same chapter:-
Seems to me you don’t want to get ’Seph out of bed for this. She’s still jet-lagged from that caper in Queensland last Thursday.”
Vikram shakes his head. “Then we shall let BASHFUL INCENDIARY sleep in,” he agrees. Sending one of the Laundry’s most powerful witches on a wild goose chase is a bad idea: Persephone has a vicious temper when sleep-deprived.