There are some established patterns for S3 as a database. It's extremely common in "data lakes" (throw data of various schemas in and use a tool that can parse at query time).

There's client libraries like Delta Lake that implement ACID on S3.

Much of the Grafana stack uses S3 for storage (Mimir/metrics, Loki/logs, Tempo/traces).

That said, I'm not sure about the implementation Tarsnap uses--if it's completely ad-hoc or based of other patterns/libraries.