This a huge departure from the original plan. We weren't going to see Izzy anymore after Chapter 5. Or rather we wouldn't have when it was still Chapter 10 of Book 2. Initially, Izzy was going to go full villain and beat the crap out of Evey. We had more time leading up to that event so it made more sense. Then, Izzy would've stopped herself and snap out of it only to be literally erased from existence from reality. In Book 3, Evey was only going to have the faintest of memories of Izzy if they had them at all. She would just be a nagging notion in Evey's mind. The universe had done a bad photoshop job of removing Izzy.
My editors insisted I not kill Izzy and to drop Book 3 completely and come up with a new second half of Book 2. So I complied. Now Izzy's alive. And I'm glad she is. I was really bothered by how Izzy went out in the original plan. It felt right but also felt wrong at the same time. Now everything feels more correct. Izzy gets to atone and live to see another day.
So enjoy Izzy in her new situation. We almost lost her.