While on iOS 10 deleting stock apps only hid them, it seems like on iOS 11 (since beta 1) they are actually deleted just like any other app. I haven't tried the betas yet, but users from different sources can confirm. It would be interesting to investigate the technical implications of this. Maybe non-essential system apps are now installed on the user partition as normal App Store apps? If this is the case, the system partition could be made smaller, and more free space would be left to the user. Someone would need to explore the root filesystem in order to confirm this. Also, how does iOS react to this change? What happens when you tap an e-mail address but the Mail app is no longer installed? Does a third-party app get opened?