Microsoft announced that they are removing Basic authentication access support for EWS on October 13, 2020:
Similarly to how we dealt with Apple changing to required app-specific passwords in 2017, we will aim to make this transition as seamless as possible.
During 2019 we will support authentication via OAuth for the situations that do not do so already. Following that we will alter our authorization screens to encourage the use of OAuth.
Early in 2020 we will inform anyone using Basic authentication the steps they need to take to switch to OAuth authentication, and doing so should be seamless as they will be replacing their working Basic authentication credentials with working OAuth authentication credentials and so be no point at which there is not an active connection to their calendars.
We will also remove the ability to use Basic authentication at all to prevent anyone going back to Basic authentication around this time.
With Apple we sent out 3 rounds of notifications to people affected by the change, and we would likely do something similar here.
We will also liase with our customers as to who will be responsible for communicating with those people affected as we know some customers prefer to be the ones to communicate with their customers when possible.