When you disable modern authentication in Exchange Online, Windows-based Outlook clients that support modern authentication use basic authentication to connect to Exchange Online mailboxes. For more information, see How modern authentication works for Office client apps. When you enable modern authentication in Exchange Online, Windows-based Outlook clients that support modern authentication (Outlook 2013 or later) use modern authentication to connect to Exchange Online mailboxes. Modern authentication is based on the Active Directory Authentication Library (ADAL) and OAuth 2.0. Modern authentication in Exchange Online enables authentication features like multi-factor authentication (MFA), smart cards, certificate-based authentication (CBA), and third-party SAML identity providers. That are not yet there in new EAC at Other Features or use Global Search that will help you Other admin centers and remaining ones will soon be migrated to New EAC.

While most of the features have been migrated to new EAC, some have been migrated to Microsoft recommends using the new Exchange Admin Center, if not Effective from December 2022, the classic Exchange Admin Center will be deprecated for

Exchange online modern authentication