It doesn't matter if your infra is in another region, because there will almost always be transitive dependencies on us-east-1. IAM is deployed in us-east-1 and there will always be a transitive dependency on us-east-1
I don't know what the architecture of IAM looks like, but somehow it's never suffered a global outage.
AWS is really, really good at regional isolation.
Authentication possibly, but the control plane has gone down preventing changes.
Not being able to update your existing resources is still an outage from a DevOps perspective.
It might be an API level outage vs an end-user level outage from your customer's perspective, but if the functionality is down, it's an outage.
Our data plane was fine (for example, ec2 instances and s3 buckets in other regions were fine).