"Any Microsoft 365 service that leverages Azure Active Directory (AAD) authentication may be impacted by this issue. We've identified and are reverting a recent change to the service which may be causing or contributing to impact."
These types of outages are thankfully rare, but it always shocks me just how unexpectedly fragile our modern communication really is. Sure, there are other email services, but the impact this has is nonetheless surprising to me.
https://portal.office.com/servicestatus is starting to get populated.