According to the teardown of the Play Store APK, Rahman found more code that references Instant Apps. While it doesn’t tell us when exactly the feature will be going live, it does show that work is progressing on the feature and with Google I/O 2017 coming up, perhaps Google might take the feature live then.
For those who are unfamiliar with Instant Apps, basically instead of users having to download an app just to use one or two features, or just to use it temporarily, users can launch the app fully functional from the web and use it like a normal app until they are done. There are advantages to this, such as saving on data where it could be less data intensive to use the app on the web as opposed to downloading it.
It also makes it a bit more convenient as it lets users try out apps before deciding if they want to download it. In any case hopefully we’ll hear more about the feature soon.