Developers looking to get a jump on the next Android features have a small set of blessed hardware to choose from with Android P: Pixel, Pixel XL, Pixel 2 and Pixel 2 XL.
Eventually, Android P will be sent to new phones from other manufacturers, along with the handful of phones that third parties are concerned about updating, but there are some major elements of Android that will not enjoy this wonderful future: Nexus 5X and Google Nexus. 6P phones, and the Pixel C tablet often forgotten. As Ars Technica confirmed with Google, those devices will not receive Android P when it launches completely. Also, as pointed out by Android Police, there is no developer preview image for Nexus Player, which came out in 2014, so it could also be receiving updates.
It is 2018, and we are beyond the two years of important support for the operating system update that was promised to these devices, so this is not very surprising. All three devices will continue to receive monthly security updates until at least November this year, but they will remain stuck in Android 8.1 for an underlying operating system with respect to official Google updates.
This is, of course, the moment when I point out that iOS 11 works on the iPhone 5S of 2013, the iPad mini 2 and even the sixth-gen iPod touch. I should also mention that Android tablet support could hardly be considered "nominal" now that the last official Google tablet is outside the main update window.
But let's not forget that Google has done a good job of transferring the main features of its operating system to its applications and services, and the open nature of Android in combination with community support fills some of the gaps, so that Few very few phones get completely left behind these days.