Without any notification or explanation, Google has extended the security update support for the Nexus 5X and Nexus 6P an additional two months. Both phones will now receive minor software patches until November 2018.
The change is reflected in an update to the Nexus Android version page on Google Support and appears to be the only changes to the schedule which the company adopted in 2016.
In 2016, Google began dividing up the monthly security updates into two (or three) patches. Generally the first patch each month, usually dated the 1st each month, deals with Android security patches and other system wide fixes. The second patch, generally dated the release date of the patch on any given month, is more for phone specific driver fixes and is mostly aimed at the Nexus and Pixel phones. It was at that time the referenced schedule for patches was updated with major updates for two years after a phone was released and three years on security patches.
For the Nexus 5X and the Nexus 6P, that means that major updates ended in September 2017 with Android Oreo with security patched slated to end in September 2018. Now it is updated to November of next year but Google has not indicated why they have extended the updates.
Note that the change only impacts the security patches. The major version dates are still slated to end this month for those devices. That means that Oreo will be the last major version of Android for these devices.