Today Apple sent out an email to developers about the security of their accounts. The emails states:
In an effort to keep your account more secure, two-factor authentication will be required to sign in to your Apple Developer account and Certificates, Identifiers & Profiles starting February 27, 2019. This extra layer of security for your Apple ID helps ensure that you’re the only person who can access your account. If you haven’t already enabled two-factor authentication for your Apple ID, please learn more and update your security settings. If you have any questions, contact us. Best regards, Apple Developer Relations
There are a few possible reasons for this. The first is, as the email states, to help secure developer accounts. By enabling the two-factor authentication, particularly for Certificates, Identifiers, and Profiles cannot be added by unauthorized users.
This will have some downsides though. By requiring two-factor authentication, only ten devices will be able to receive the two factor authentication codes. For most individual users, this will not be a problem. Five of these trusted devices can be Macs and five of these can be iOS devices.
I contacted Apple Support to verify the number, and it is indeed ten trusted devices that can be associated with an Apple ID.
For larger development groups who may need to allow more than one user to login to the Certificates, you will likely need add a user who has access to the Developer Resources.
If you have not already enabled two-factor authentication on your Apple Developer account, you will want to review the two-factor authentication support page to be sure that you have a way to recover your account, if needed.