After seemingly ignoring the situation for years, Microsoft delivered modern authentication for Exchange Server (for pure on-premises organizations) in Exchange 2019 CU13. The solution uses ADFS to issue and manage the OAuth 2.0 tokens and is supported by the latest version of Outlook for Windows. Support for other clients is in the works.
From November 1, 2021, Microsoft requires Outlook 2013 Service Pack 1 (with fixes) as the minimum client version to connect to Exchange Online. Given all the publicity about attacks against the on-premises version of Exchange earlier this year, it's a wonder why organizations continue to allow people to use outdated client software to connect to Exchange Online. In any case, the drop-dead date is November 1. If you have any old Outlook 2007, Outlook 2010, or Outlook 2013 (before SP1) clients, it's time to start upgrading.
