Starting December 1 2020, Yammer Groups API endpoints will only support the usage of Azure Active Directory (AAD) tokens. Yammer Groups API endpoints will no longer support the usage of Yammer OAuth tokens. Microsoft recommends that customers and partners transition to using Microsoft Authentication Library (MSAL) and AAD tokens with the Yammer API.
Once this change goes into effect, using Yammer OAuth tokens will result in a bad request response from the server. In order to avoid this, Microsoft recommends that you transition your apps to authenticate using the MSAL to acquire appropriate AAD tokens from the Microsoft Identity Platform and set up an AAD client application.
Read more: https://bit.ly/31POtub
Posted by Jason Bell, Strategist and Chief Technologist