A new test build of Android O has shown up in Google's code commits, with "Shamu", Motorola's 2014 Nexus 6 shown as the device that the build is intended for. While the Nexus 6 has officially reached its end of update life, it was still set to receive security updates. The presence of an Android O test build stands in contrast to Google's policy of focusing on newer devices, but it's quite possible that Nexus 6 owners will never actually see this build hit their device. Regardless, it's currently unclear whether this test build is just made to test libraries present in the Nexus 6 or if it's being made with the intention of eventually letting Nexus 6 users run a stable release of Android O.
Looking over the files in the tree, the build seems to be in a very primitive state at the moment; the proprietary elements are all there and most of the basic hardware is accounted for, but the Nexus 6's unique handling of critical thermal situations doesn't seem to be coded in, and hardware acceleration using the phone's Adreno 420 GPU is also nowhere to be seen. Network hardware, meanwhile, seems to be running on generic drivers; since the Nexus 6 was infamous in its heyday for not working correctly on certain carriers' LTE networks, such as T-Mobile, it remains to be seen whether Google improves that aspect of the device. Still, most of the basic elements seem to be there, so enterprising ROM builders can always take the existing hardware-related code from Nougat that's missing from the Android O build and see how well it all works when combined together, though that method historically yielded less than stellar results.
It should be noted that the Nexus 6 has yet to receive a build of Android 7.1.2 Nougat, which is the version that current Nexus devices are on, as well as Google's newer Pixel lineup. Google is often seen testing new software on old hardware just to check compatibility and doesn't always release the results of that testing. As mentioned above, the results do end up in Google's Android Open Source Project. While this doesn't always lead to viable builds of newer ROMs, the new elements created by Google to get old hardware working with new software can always be used to enhance Nougat ROMs and add in new features that only newer officially support.