Smart Reply could come to other chat apps via the 'Reply' app, an application which seems to be in testing at the current moment and is presumably already in use by some users. It's also possible to sign up to be a Reply tester and you can access a form to fill out to be considered by clicking on the button below. According to the form, the smart reply feature would be available for popular messaging apps like Android Messages, WhatsApp, Facebook Messenger, Line, Wechat, Slack, Skype, and Hangouts.
While there's no confirmation that Reply would add the smart reply feature to all of these apps, the form asks which popular messaging apps are used by the person signing up, and the above-mentioned options are all options which you can choose from. Also worth mentioning is that in the images in the gallery below, the two apps being displayed are Hangouts and Android Messages, so it will at least be usable by these two apps. Reply is a new application that's being tested by Google's Area 120 division, which specializes in experimental products and services.
Since this is currently in testing there's no way to get your hands on the app just yet other than through signing up to be a tester through the official form. There's also no indication of when or if Google and Area 120 plan on releasing Reply to the Play Store at some point for all users to download. According to the description for Reply, the app will add the smart reply options into your notifications that come in from the supported chat apps similar to the ones you'd see from Allo, but it's also noted that some reply options could be a "little bit smarter," such as the option for Android Messages below which gives an exact answer to the question along with a car emoji for added effect. Reply may be Google's answer to adding smart reply to more apps than just Android Messages, which some Project Fi customers should already have access to, but it's possible that Google could also roll these features into its own apps and make it possible for developers to do the same instead of pushing out a separate app entirely.