While I think it’s true to say iMessage apps are a promise that’s still waiting to be kept, I’ve little doubt we’ll see these lightweight apps become more important — and more ambient — in the future.
What are iMessage apps, and how do I use them?
iMessage apps are available from the App Store for Messages.
You find the store and access those apps you have already installed by tapping the App Drawer icon left of the text entry window in an active iMessage. Tap the App Store to find new apps. Apple includes a few default apps — Recents, Music, Digital Touch, GIF images, and Apple Pay (where available).