[selykg]

There are apps using this though. Two that ring a bell for me are Agenda and Hook though only the first one has an iOS app that also utilizes this model.

I assume they give each feature a date of availability and compare to the last purchase made via the App Store receipts and make features available or unavailable based on that intersection.

Edit: Edited for URLs, I wish this stupid site would support at least basic markdown... So mind-blowing it doesn't.