Much has been said and written about disabling Basic Authentication in Exchange Online, and for good reason. Basic Auth is insecure and makes it easy for bad guys to hack your accounts and access your organization's data.
Microsoft disabled Basic Auth for most Exchange Online protocols in October of this year. Those protocols include Outlook, EWS, RPS, POP, IMAP, and EAS. SMTP Auth was also be disabled in your tenant if it is not being used. Modern Authentication is the secure way to authenticate for these protocols. Congratulations to Microsoft for pulling off such a monumental achievement to help keep customers' data safe!
Read Deprecation of Basic authentication in Exchange Online | Microsoft Learn for more information.Next up, Microsoft is going to disable Basic Auth for the Autodiscover protocol. I would argue this is one of the most significant changes to deprecating Basic Auth, since it is continuously used by Outlook and whenever you configure a mail profile on a mobile device that uses ActiveSync. Because of this, it's easy to ignore this traffic when monitoring for Basic Auth usage. It's also a fairly easy protocol for the bad guys to use for password guessing or dictionary attacks.
As with all the other impacted protocols, Microsoft is not turning off the protocol itself, only the ability to authenticate to the protocol using nothing more than a username and password.