Just opened Drive after upgrading to a family plan and saw that I got access to “Photo Backup”. I contacted support about the iOS beta when it was announced and was told that it’s not yet available for iOS. I am using the TestFlight version but not sure when this came through.
EDIT: I added a link to a folder on Drive which contains some screenshots from the iOS app.
There’s been no announcement from them either so not sure why I got it already.
It’s a function that’s still in beta, on a gradual rollout, so here you have some more data: https://proton.me/support/enable-photo-backup