App registration in Azure AD is manditory to get the OAuth 2.0 access or authentication token to access the data in Microsoft Graph either using REST APIs or SDKs. There is a OAuth 2.0 Resource Owner Password Credentials (ROPC) grant authentication flow but it does require App ID (or Client ID).
Documentation Reference :
https://learn.microsoft.com/en-us/azure/active-directory/develop/v2-oauth-ropc#authorization-request
Hope this helps.
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