The Nexus 6 just recently reached the end of mainstream software support from Google, but it is still receiving monthly security updates. The latest OTA, which included the March security fixes , seems to have broken SafetyNet - which in turn, disabled Android Pay.
If you're not aware, SafetyNet is an API that allows applications to check if your device has been tampered with - unlocked bootloader, custom ROM, etc. If the SafetyNet check fails, several apps that require a secure device will fail, including Android Pay.
Many users on the Nexus 6 subreddit are reporting the problem, even with locked devices that previously passed SafetyNet. Theupdate has been removed from the Nexus Factory Images and Nexus OTA pages, and the Twitter support account for Nexus devices has repeatedly said Google is investigating:
@MeKevinB Hi Kevin. We're aware of this issue and our team is investigating. Thanks for your patience!
― Nexus (@googlenexus) March 9, 2017