Earlier in September, Google's contextual assistant feature, Now on Tap, became available to those running the Android 6.0 Marshmallow developer preview after a Google app update, but it disappeared shortly after. Following Google's event earlier today, where it once again showed off the feature on stage, it appears that it is now working again for those on the developer preview.
To recap, Now on Tap works by predicting what users want to know and serving up Now cards based on the app you're currently using. It is accessed by long-pressing the home key, at which point it will pop up with useful cards based on aspects of what you're looking at in an app, similar to what you've seen on the Google Now screen.
It's likely that the disappearance of the feature was due to some weirdness that was occurring just after it became available, with a number of users reporting errors and app crashes. However, it looks like those running the Android 6.0 developer preview can once again start testing the feature out in full. As for the rest of us, we'll have to wait for the update to be released en masse.
Thanks for the tip, Tom!