"Every day Basic Authentication remains enabled in your tenant, your data is at risk, and so your role is to get your clients and apps off Basic Authentication, move them to stronger and better options, and then secure your tenant, before we do."
Microsoft Exchange Team
Back in 2019 Microsoft announced they were turning off Basic Authentication for all Exchange online tenants in Microsoft 365. Then on October 2020 this was suddenly put on hold due to COVID and the impact it would have on business. Since then Microsoft have been turning off Basic Authentication on Tenants that have not been using it without impacting the end user. Now Microsoft have now set a new date, announcing that on the 1st October 2022, Microsoft will begin to permanently disable Basic Authentication in all tenants, regardless of usage. IMPORTANT DATES
October 22, 2019 – Security Defaults are now turned on by default for all new users. Security Defaults block all legacy authentication protocols.
October 13, 2020 – the initial date for disabling basic authentication in Exchange Online for all tenants. (Postponed)
October 2020 – basic authentication will be disabled for tenants which do not effectively use it.
Second half of 2021 – that’s when basic authentication will be disabled for all tenants. More precise date is yet to be announced. (Postponed)
February 2021 – Microsoft announces that basic authentication will not be blocked for now for any protocols that a tenant is using. However, basic authwntication will be blocked for the unused protocols, with a warning issued 30 days beforehand in the Microsoft 365 Message Center in your tenant
October 01, 2022 – the complete shutdown of basic authentication for connections to Exchange Online, announced in September 2021. That should be more than enough to tie up all the loose ends.
So if you haven't already made the switch to Modern Authentication, now is the time to be proactive and start planning and implementing the change in a coordinated tested method before Microsoft makes the change for you. For more information please read the Microsoft Exchange Team September 2021 Blog Update.