G Suite users will be able to log in using security codes. A security code is a one-time use code, generated using a security key, that can be used on legacy platforms where security keys arent supported directly. Security codes will be available by default for some users: If you currently use an only security key policy and wish to allow security codes, an admin can choose turn security codes on for specific users (see more below). Security codes increase account security significantly. With this launch, users can now generate a security code with their security key which can then be used to authenticate their login attempt.”]
Source: https://workspaceupdates.googleblog.com/2019/06/security-codes-and-security-keys.html