Accodring to a Tweet from Stephen Hall, the 2015 Smartphones Nexus 5X and 6P will geht the Google Assistent.
Stepehen Hall , from 9to5Google, seems to have a neverending amo0unt of leaks. In his latest tweet he wrote that in a next major Update the 2015 Google Phones Nexus 5X and Nexus 6P will get the new Google assistant. When this happens, those phones will be 2 out of 3 non-pixel devices that feature the googlke Assistant. The third one is rumored to be th LG G6.
It has not yet been confirmed which Update is meant, it could be Android 7.2 or we even have to wait for Android O. Maybe we will hear something at MWC 2017.