From bleepingcomputer.com
Users who are part of the Apple Developer program have started receiving emails that state they need to add 2-factor authentication to their accounts by February 27th, 2019. Otherwise, they will be locked out of their Developer accounts and be unable to access their Certificates, Identifiers, and Profiles.
The emails being sent have a subject line “Two-Factor Authentication Required Soon” and state that they are requiring two-factor authentication in order to provide an extra layer of protection for a developer’s Apple ID and to make only they have access to their account.