Google released the first developer preview for Android P today, and it is only available for the Pixel, Pixel XL, Pixel 2 and Pixel 2 XL devices. Which means all devices that were released before 2016 are no longer supported - that also translates to meaning, no more Nexus support. Now this can mean that the Nexus 5X and Nexus 6P are no longer getting major updates, or it could also mean that it'll see an official Android P update in the fall once the update makes its way out of a preview. That has happened in the past, but it usually means that neither device will get Android P, officially. Of course, there will be plenty of ROMs available from developers for both smartphones in the coming months.
Android P is getting an early preview again this year, similar to Android N and Android O in the past two years. And a stable release will be coming out around August or so. Which is before the Pixel release - if Google keeps up with the schedule it has used in the past two years. Having the earlier previews means that developers are able to get their hands dirty in the update ahead of the stable release. This allows Google to get feedback on things, and work out some bugs, as well as allowing developers to get in there and update their apps to support the new APIs and such.
With Google dropping support for the last generation of Nexus devices - which debuted in 2015 - that means that the Nexus line is officially done. It shouldn't be a huge surprise to see that the Nexus 5X and Nexus 6P are not getting support for Android P, seeing as they are over two years old, and Google only promises major updates for two years, and security updates for three years. Of course, it's good to see companies going above and beyond, but Google did keep its promise here. As for the reasoning behind the Nexus 5X and Nexus 6P being left out, Google likely will never make that public. But it likely has to do with the hardware that is inside the devices. Which are using pretty old chips now in the Snapdragon 808 and 810 respectively. In order for Google to still support those devices, Qualcomm needs to be supporting those chipsets as well. So there's a lot of moving parts here, and it's unclear what the actual reason is for why there's no Android P coming to the last-generation Nexus smartphones.