For a long time now, Google has made factory images available with each new system update for Nexus devices, but the problem was, you had to have an unlocked bootloader in order to flash them. Not anymore. Starting today, Google now offers full OTA images for all currently supported devices. These are not the incremental OTAs which sometimes take weeks to fully roll out. These are full OTA images that you can simply sideload with adb in recovery regardless of whether your bootloader is locked or not.
There appears to be one more "feature" that has been thrown in for good measure. Namely, you now have to agree to terms and conditions before getting links to either factory images or OTAs.
Google initially started this full OTA business with the N preview to fix some soft-bricking issues and also to get you back to Marshmallow if you opted out of the beta program. It's nice to see the company roll this out retroactively for new 6.0 builds as well. There are a lot of reasons, chief among them security, to keep your bootloader locked, so it's great that El Goog has finally recognized this.
- Google Developers