Back in them good ol days Android was actually multi user, like any other Unix. I had it working on some old stock Android, I recall.

I would be surprised if it still worked 😅

Not just GrapheneOS. Any Android phone has app pinning.

And for the camera roll problem. I use Ente Photos and the app has that. You can select a range of photos and tap "guest mode". Then trying to scroll outside the selected photos requires to unlock the phone.

I played around a lot with pinning apps protect the rest of the phone from my son when he wanted to use an app. My genius of a son somehow always found a way around and managed to access other parts of the phone (Samsung). I never relied on app pinning as rock solid security.

iOS features a password or biometric lock protected “hidden” photo album. Photos placed there don’t show up anywhere else as far as I can tell. I haven’t find any type of app lock yet, but I would think it does exist somehow, as other features can be accessed and used while the phone itself is locked. My point is the ideas mentioned are all easily implemented, and should be more obvious and ubiquitous.