Google Pixel and Pixel XL will get software updates at least until October 2019
As you can see below, Google can't guarantee that the Pixel and Pixel XL will get Android version updates after October 2018 (thus two years after their initial launch). It also can't guarantee that the two devices will receive security updates after October 2019 (3 years after launch). To be fair, this shouldn't surprise anyone. Android version updates for 2 years and security updates for 1 extra year have been the norm for Google when it comes to Nexus phones, and it's only natural for this to continue with the Pixel series.
source: Google via Android Police
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Things that are NOT allowed: