We'll have to wait for the postmortem, but I bet it was an unintentional dependency on S3 that no one realized had come into place until S3 went down -- especially considering how fast they were able to remove the dependency and fix it.
This reminds me of a GitHub outage from them having a build dependency on GitHub. IIRC, they tried to roll back to building a prior version but since the site was offline, the build failed.
S3 gets used to store a lot of static content. Can't speak for that team, but I'm sure they'll take that feedback. Happy the banner functionality remained unimpeded.
