Microsoft has announced that Exchange Web Services (EWS) on Exchange Online will no longer get any further updates and Basic Authentication for EWS will be decommissioned.

NOTE This does not apply to Exchange On-Premises (for now ?)

Microsoft has been heavily investing in the Graph technology and it is highly recommended to start using/migrating Graph for your interaction with Exchange Online, in lieu of EWS.

In parallel, because we all know this is a weaken authentication mode, Basic authentication will be decommissioned for EWS on October 13rd, 2020. So you also have to start using OAuth 2.0; you can take a look at https://developer.microsoft.com/en-us/graph/docs/concepts/auth_overview to start using OAuth with Graph or https://docs.microsoft.com/en-us/exchange/client-developer/exchange-web-services/how-to-authenticate-an-ews-application-by-using-oauth if you have to continue to use EWS for your on-premises environment.

You can get the complete announcement here https://blogs.technet.microsoft.com/exchange/2018/07/03/upcoming-changes-to-exchange-web-services-ews-api-for-office-365/