Bareos offers different approaches for S3-compatible storage. Hetzner has recently published a guide on how to set up Bareos with Hetzner Object Storage. The guide covers installation, configuration and backup setup using the Bareos dplcompat Storage Backend.
Read the Hetzner guide: https://docs.hetzner.com/storage/object-storage/howto-backups/bareos
While the Bareos dplcompat backend uses S3 as storage target, Bareos also supports backing up data stored in S3 with the including the Apache Libcloud Plugin, which gives users additional flexibility depending on their environment and workflow.
For more details on plugins and S3-compatible backends, please see the Bareos documentation.