Azure AD has a history of outages which have caused problems for Microsoft 365 tenants over the years. Microsoft hopes to solve the problem with a backup authentication service that’s capable of keeping things going if the primary Azure AD service goes offline. Basically, the backup service has copies of successful authentications over the last three days which can be used to process authentication requests for most sessions when the primary service fails. It seems like a good idea.
https://office365itpros.com/2021/11/25/azure-ad-backup-authentication-service/
