The idea that AWS's services are fully regionalized or isolated has always been a myth.

All the identity and access services for the public cloud outside of China (aka "IAM for the aws partition" to employees) are centralized in us-east-1. This centralization is essentially necessary in order to have a cohesive view of an account, its billing, and its permissions.

And IAM is not a wholly independent software stack: they rely on DynamoDB and a few other services, which in turn have a circular dependency on IAM.

During us-east-1 outages it's sometimes possible to continue using existing auth tokens or sessions in other regions, while not possible to grant new ones. When I worked there, I remember at least one case where my team's on-calls were advised not to close ssh sessions or AWS console browser tabs, for fear that we'd be locked out until the outage was over.