Here’s what the Apollo dev said about how he implemented it: https://old.reddit.com/r/apolloapp/comments/14lm3k4/i_just_released_an_important_apollo_update_that/jpzxc0f/

I can do a blog post something. I basically reimplemented that aspect of the navigation controllers and keep popped view controllers around in memory until the user pops something else on top of the ones that were kept around, that way you can always return. The tricky part was recreating all the animations and interactions to make it feel like it was still the iOS interaction.