Google is not launching an official Android 7.0 Nougat for the Nexus 5, but that doesn't mean others won't do it for them.
The Nexus 5 was the most successful Nexus device so far, and it was a sad day when Google hinted that the Nexus 5 wouldn't get Android 7.0 - now confirmed by the official Android Nougat launch. But, as any open-source project, it didn't take long for someone to take the job into their own hands, and so... there's already a Android 7.0 for the Nexus 5, provided you're willing to take some risks.
For now this version "works", but there are things that still need to be fixed (like the camera and bluetooth). So, until it gets a bit more polished and bug free, it's not the sort of system you'd like to have on your main device. But, considering the Nexus 5 is already showing its age, it's highly likely many owners have already moved on to newer devices (or about to do so, when this year's Nexus line is unveiled), and then they'll have more freedom to use it as a test device.
If you're interested just head over to XDA and check how to get Android 7.0 Nougat on your Nexus 5.