Google wants your Nest secured with two-factor authentication
The security feature was announced earlier this year, and Google will start enforcing it this month. If you don't want to deal with emails, every time you log into your Nest account, there are two alternatives. You can consider migrating to a Google account, or you can enable two-factor authentication.
Judging by the comments under the post, users are not happy with the impending change. Some of them feel that adding two-factor authentication to a thermostat is overkill, while others outright say that "this will not work." It's unlikely for Google to have a change of heart, though. The company is advising Nest owners to make sure they have access to the email they use for Nest.