Nexus SharePoint on-premise will reach its 'end of life' in April 2023, at which point the service will be switched off and only SharePoint Online will be available.
If you use SharePoint on-premise, you will see a new login page from 27 January 2022. There will be no changes to the username format or password - these will remain the same (Oxford username + Single Sign-On password). The login page looks like the image below:

SharePoint on-premise NEW login screen