Pixel security patches aren't guaranteed after October of 2019. This isn't necessarily the last month that Google will send out a security patch for the Pixel and Pixel XL, but up until then Google is guaranteeing a new security patch every single month, and after that point devices could be left out, and it's likely that they would be as Google rarely ends up pushing any sort of update out to devices beyond the dates they have set for a cutoff. As has always been the case, Pixel and Nexus devices are guaranteed security updates for a three-year period from when the device first made available for purchase from Google, though Google has amended this to be 18 months after the last time devices were sold even if the 18 months is further out than the three-year period.
In addition to listing off when security patches are expected to be cut off, Google has also listed when Pixel devices can expect to be locked out from software version updates, which is after October of 2018. This would mark the two-year period of Google's software update guarantee from when the device was first made available and similar to the security patch guarantee this is nothing new. This means users with a Pixel or Pixel XL can certainly expect Android O as a version update, and whatever the next version after it is that will be coming in 2018. After that, chances are they wont' get any additional new software officially through Google.
Nexus devices are listed on the same page as the Pixel and Pixel XL, and this includes the Nexus 6P and Nexus 5X, the two most recent Nexus devices and both of which were launched in 2015, as well as the Nexus 9 and Nexus 6 which are the only other two that are still guaranteed. The list also shows Nexus devices dating back to the 2012 Nexus 7, but anything before the Nexus 6 and Nexus 9 is no longer supported. According to Google's list, the Nexus 9 and Nexus 6 are not guaranteed any security patches after October of 2017, while their Android version update guarantees already expired in October of last year. As for the more recent Nexus 5X and Nexus 6P, version updates are no longer guaranteed after September of this year, while security patches will not be guaranteed after September of 2018. If you own a device which is still supported by either software updates, security patches, or both, you can see the full list in the images down below.