Google introduced Smart Replies with its short-lived Reply app over a year ago. Since then, the tech has been added to more prominent apps from the company such as Messages, Gmail, and Hangouts Chat. The Smart Reply API was only made available to third-party developers a few days ago, but Klinker Apps has already gone ahead and incorporated it in the latest beta of its popular Pulse SMS app.
For those unfamiliar with Smart Reply, it essentially suggests words that it thinks are appropriate responses for received messages. Pulse SMS's v4.8 beta is among the first non-Google apps to use the API, and it seems to work decently well. Oftentimes the Smart Reply suggestion bar wouldn't pop up when I wanted it to, but I'm not sure if that's an issue with the implementation or just the nature of how the API works.
Pulse has also built in the ability to customize if/where Smart Reply suggestions show up in the notification actions. For instance, I have the "Suggest Smart Reply" option in the second slot, so it'll show up as the second notification action when needed. Of course, if you're not a fan of Smart Replies in general, you can disable the whole thing altogether.
We have the Pulse SMS v4.8 beta APK over on APK Mirror for download if you need it. And if you're not a Pulse user, you should consider switching over; it's highly customizable, it gets the latest features quickly (as seen here), and it's great for those who text from multiple devices.