it was already announced on the Microsoft blogpost New minimum Outlook for Windows version requirements for Microsoft 365, Microsoft will stop support for older Outlook clients on November 1, 2021 (which is 24 days from the time of writing!).
In short, all clients older than Outlook 2013 SP1 with the latest fixes are no longer able to connect to Exchange Online. And yes, this includes Outlook 2010 (I know there are still clients out there running Office 2010!).
More detailed version numbers of Outlook that will not connect anymore:
|Office version||Outlook version|
|Office 2010||All versions|
|Office 2013||15.0.4970.9999 and older|
|Office 2016||16.0.4599.9999 and older|
|Office 365 ProPlus||1705 and older|
To check the version of Outlook you are using, select File –> Office Account –> About Outlook. It will show something like Microsoft® Outlook® for Microsoft 365 MSO (16.0.14326.20384) 64-bit.
Please be aware that this is completely independent from the Basic Authentication strategy that Microsoft is following. Older versions will just stop connecting to Exchange Online.
For more information, check MC288472 in the Microsoft 365 Message Center.