As part of the ongoing effort to improve the security of Android, monthly security updates are provided for each device to include the latest security patches so you don’t have to wait for a major release.
These updates are only provided for a certain amount of time before it is considered you should really be upgrading to a new device given the age.
Google has updated its support page that lists when it will stop providing updates for the Nexus and Pixel devices. Google specifies that it won’t guarentee regular updates will be provided for current Pixel devices after October 2019.
The Nexus 6 and Nexus 9 will cease to get updates as of October 2017, while the 5X and 6P will see updates until September 2018.
This does just cover security updates that are provided monthly so Google may still provide adhoc patches that it sees fit for these devices past this date.
