Similarly to how Google tests new features in select markets (typically the United States) before rolling them out to more countries, they test new software features on their own hardware before rolling them out to more devices. A perfect example of this is how Google Assistant was a Pixel-exclusive software feature at first so they could test it on a select number of people. It then began rolling out to other devices once they were confident it was ready. Some engineers also tested a feature on the Pixel 2 that would automatically enable Do Not Disturb Mode when you're driving, but that feature has now evolved into what Google is calling "Driving Mode" and it is showing up on multiple non-Pixel devices right now.
Automatic Do Not Disturb while Driving feature rolls out for some users
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