After years of compatibility, Microsoft devs have decided to drop Internet Explorer 11 support for various Microsoft 365 applications, starting with Teams. This announcement was first posted on the Microsoft 365 blog.
The new changes go into effect on November 30th, 2020 beginning with Microsoft Teams. By August 17th, 2021 (a year to-the-date from the announcement) all of the remaining Microsoft 365 apps will no longer provide support for IE 11.
What does this mean for IE 11 users? Anyone using Internet Explorer 11 to connect to Microsoft 365 apps after the dates listed above will encounter serious degradation in performance. Some features may stop working entirely.
While users can no longer use IE 11 for these apps and services, they can use Microsoft Edge's Internet Explorer mode. It's not clear if the long term goal is to migrate users away from IE entirely, but the drop in support for Microsoft 365 apps is a step in that direction.
Microsoft iterates in the announcement that Internet Explorer 11 isn't planned to go anywhere.
In the meantime, Microsoft 365 users should probably go ahead and make the switch to Microsoft Edge. The Chromium-based version is clearly the future, as Microsoft is sunsetting the Microsoft Edge Legacy app on March 9, 2021.