Google’s gone public with the month and year your Pixel or Nexus device will be cut off from receiving security updates. This is different from OS upgrades since the company generally stops the latter before the former.
Google generally pushes out new Android versions and security patches for at least 2 and 3 years, respectively, after a Nexus or Pixel product first hits the Google Store. Security updates could also end 18 months from when the Google Store last sold the device, depending on whichever period is longer.
This system makes things a bit confusing since it’s not exactly a precise timeframe which you can keep in mind when you own a Nexus or Pixel or are thinking of buying one. Posting a precise month and year timeline for all current devices is a lot simpler, although it should be noted that Google has posted the minimum update period so upgrades may or may not continue after that.
The Pixel and Pixel XL are set to get guaranteed Android version updates until October 2018 and security updates until October 2019. The Nexus 5X and Nexus 6P have until September 2017 and September 2018 for OS and security promotions, respectively.
The Nexus 9 and Nexus 6 have already sailed past their OS update eligibility period, but the two are still supposed to receive security bundles till October 2017. The latter notably got updated to Android 7.1.1 Nougat back in January this year after a 1-month delay.
You can check out Google’s upgrade timeline here.