When is the most appropriate time to perform a web service check to see whether or not the current version is still supported?

1. How does web service authentication work?
2. How do I know if modern authentication is enabled?
3. How do I know if Outlook is using modern authentication?

How does web service authentication work?

The Web Services Security message receiver authenticates the sender by validating the user name and password against the configured user registry. With the LTPA method, the sender attaches the LTPA BinarySecurityToken it previously received in the SOAP message header.

How do I know if modern authentication is enabled?

In the Microsoft 365 admin center, go to Settings > Org Settings > Modern Authentication. In the Modern authentication flyout that appears, click to enable or disable Turn on modern authentication for Outlook 2013 for Windows and later (recommended).

How do I know if Outlook is using modern authentication?

Press CTRL, right-click the Microsoft Outlook icon in the system tray and click Connection Status. Look at the Authn column. The value should be Bearer*, which means Outlook is now using modern authentication with the OAuth2 Bearer token.