Anybody familiar with the wild world of Android hackery that is XDA Developers, probably already knew to watch for exactly this sort of thing, and that it wouldn't take terribly long. When Android 7.0 Nougat officially dropped on Monday for Nexus devices, a few were missing from the list. Although their hardware was perfectly capable, the Nexus 7 2013 and Nexus 5 were officially out of the running for an official Nougat update from Google, although they will likely still receive monthly security patches for a while. Hammerhead owners stuck on Android 6.0 Marshmallow didn't have to wait long for somebody to do what Google wouldn't, of course.
A blindingly quick two days after the official launch, Santhosh M over at XDA Developers has a port of Nougat ready for brave Hammerhead users who are starting to get sick of Marshmallow. The build boots and runs decently enough, but users who don't have another device may want to hold off on booting into TWRP. The basics work well enough for the ROM to be a daily driver in only the most basic sense. You have RIL connectivity, Wi-Fi, basic audio, touch, and the display drivers, but that's about it. A long list of elements of the phone are currently non-functional in the ROM, including fairly basic stuff like the camera and Bluetooth, though the full list is not shown on the XDA posting.
With the ROM being built directly from the AOSP source, not only is it practically inevitable that the Marshmallow drivers will be reverse-engineered for full function in Nougat ROMs before long, but it's also practically inevitable that there will be other Nougat-based custom ROMs in the near future. While official CyanogenMod 14 compatibility when it drops isn't exactly guaranteed, it is quite likely; the Nexus 5 has a large and passionate developer community that dwarfs most non-Nexus, non-Galaxy phones, and even outnumbers and outdevelops its larger sibling, the Nexus 6, which has received official Nougat. The Nexus 5 is still a pretty great phone, even compared to modern flagships, and will likely end up kept alive for quite some time by the development community. For now, the bravest of the brave and those whose Nexus 5 units are expendable can head through the source link and give the ROM a try.