I believe the Nexus 6 is too old to get any future major updates. Last I read, they pulled the 7.1.1 update, downgrading these users to 7.0. It appears that you can still sideload 7.1 onto a Nexus 6, but will not currently receive OTA updates.

https://9to5google.com/2017/03/15/nexus-6-android-7-1-1-down...