When Google finally announced the Nexus 6 and the Nexus 9, they did so whilst keeping the Nexus 5 alive, sort of. The Nexus 5 didn't get a revamp or anything like that, the internals and overall design stayed the same as it had done since its launch last Fall, but Google continued to sell it. Personally, I thought this was a smart move by Google, it costs a lot less than the Nexus 6 (perhaps confusingly so, depending on what you think to the Nexus 6) and it's a much more manageable form factor for those used to smaller handsets. Alas, it seems like the Nexus 5 has seen its final days, as reports are that Google has now stopped production of LG's last Nexus smartphone.
According to TechRadar, Google has confirmed that the Nexus 5 is indeed no longer in production. Which means the only Nexus 5 devices available to be sold are those still lying around in warehouses, meaning that Nexus 5 stocks will eventually dry up, and the only option for someone looking for a smaller Nexus smartphone would be used Nexus devices. This is a little disappointing to see, as the Nexus 5 could easily keep selling alongside the Nexus 6, but we doubt that this was entirely Google's decision. It's perhaps more likely that the contract between LG and Google concerning the Nexus 5 has finished, and LG is no longer under obligation to produce new units.
Right now, the warning from Google seems to be "once they are gone, they're gone, so we'd suggest people get them while they can", and as of writing there are some still available from Amazon, but none of them are new, sealed units. You're more likely to get a better deal buying used, depending on their condition of course, but we'd imagine that everyone one who wanted a Nexus 5 already has theirs. It's worth remembering that just because production has stopped it doesn't mean that the device won't see any further updates, the Nexus 7 (2012) recently got updated to Lollipop and we'd imagine the year old Nexus 5 to get a couple more updates at least before it's forgotten.