On December 4th, 2020, the login.microsoftonline.com endpoint will be deprecated for accessing B2C tenants. Existing tenants will need to move onto B2Clogin.com before that date, and all functionality will remain the same on the B2Clogin.com endpoint.
Source: Azure Roadmap
Source Link: Azure Active Directory B2C is deprecating login.microsoftonline.com
