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.
Google improves that aspect of the device. Still, most of the basic elements seem to be there, so good 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.
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.