It seems the latest security update (March 2017) for the Nexus 6 is likely to have been pulled back by Google. The information comes after various reports have now started to surface on social media detailing that there is an issue with the update and Android Pay. More specifically, it seems the issue is with the update and the SafetyNet API, which among other things, underlies part of the running of Android Pay on compatible devices - such as the Nexus 6.
At present there does not seem to be any formal confirmation from Google that the update has indeed being pulled. However, as Android Police has pointed out, it does seem as though the updates are no longer listed on the Android factory image or OTA update pages. Which does in itself act as some form of confirmation, due to the inability for users to be able to directly pull the update. Furthermore, the Nexus account on Twitter has been responding to some questions from users and specifically stating that they are aware of the issue and they are investigating it. The assumption there being that once a fix has been resolved the update for the Nexus 6 will become available again, and likely in an amended form.
Some Nexus users will already be very familiar with SafetyNet, as reports did surface about this particular API not long after Android Pay first became available. The reason being is that SafetyNet essentially looks to identify any tampering with devices. Which did mean that rooted Nexus users were unable to make use of Android Pay - due to SafetyNet identifying rooted Nexus devices as ones which had been ‘tampered’ with and automatically disabling Android Pay. It is unclear at the moment what the current issue is with SafetyNet, although the user reports do seem to be confirming that SafetyNet checks are failing on devices that have not been tampered with, following the update. As to be expected, once Google has finished its investigation, firmer details on the likely reason for the issue, as well as details on when the update will become available again, will be made known.