In Play Store there are available two launchers made by Google - Google Now Launcher and Pixel Launcher. Second of them is used in new Pixel phones, and Google decided to stop supporting the other one, previously used in Nexus devices. Reason is simple - company wants to focus on their new phones from Pixel series, instead of developing Nexus phones.
Google Now Launcher probably will be removed from Play Store on 1st March. If you have installed this launcher on your device, it will still work, but you can't expect any big updates. There is possibility, that Pixel Launcher will be added into Play Store for every device and everyone will be available to download it their phones, but for now it's only rumor.