On Android (don't know about iOS) you can take photos without even unlocking - double press on the power button opens the camera. You can't access anything else (including existing photos in the camera roll).
It works the same way on iPhones. The lock screen includes a camera button. When tapped, the phone enter a camera-only mode in which only photos taken during that session are accessible.
You can also swipe the right on the screen to open the camera instead of pressing the camera button on the screen.
True, and it is a step in right direction. However I still don't want to expose my bank app credentials every time I show someone my vacation photos.
Why would your banks account access not require additional credentials beyond what gives access to your photos? I have an iPhone. TouchID/FaceID can give access to the photos, but to get to my bank account, I use a separate login.
You're looking for "screen pinning" on Android or "guided access" on iOS. It's been around for quite a while.
you definitely don't need to unlock to take photos on iOS.