It's basically a short game built around a "message".
I don't want to be too Italo-centric here, but many short works by Molleindustria qualify as playable leaflets IMHO. I made up this concept while talking about my Two Interviewees during a presentation for the Game Design Week in Milan (in Italian: http://www.maurovanetti.info/?q=ludovolantinare).
Some great examples:
- https://colestia.itch.io/crisis-theory
- https://molleindustria.itch.io/nova-alea
- https://sectordub.itch.io/passengers
- https://dragonlove.itch.io/gerrymandering
- https://tselmek.itch.io/the-aftermath
I don't know if the category has some academic depth, but this game would definitely fit in it. :-)