Google has followed the same device support pattern for years - two years of major Android OS updates, and three years of security updates. You might get Android updates after that, and you might get further security updates, but it's not guaranteed. Seemingly out of nowhere, Google has extended the security update period for the Nexus 5X and 6P to November 2018.
Previously, both devices were slated to receive security updates until September 2018, which would have been about three years after they were released. The support page for Nexus device updates has been changed, and now the 5X and 6P have November 2018 as the cutoff date. There's no clear reason for this change, and the end of guaranteed major Android updates is still listed as September 2017. The cutoff for guaranteed telephone and online support has also been pushed to November 2018.
We've reached out to Google asking for the reason behind this change, and we'll update this post when they respond. There's certainly no reason to complain about extended support, but with several major hardware problems cropping up in both devices, I'm not sure many 5Xes and 6Ps will even make it to late 2018.
A Google spokesperson told us, "we don't have anything to add beyond what's on the page." So the reasoning remains a mystery.
- Google Support