Note that I have the "-Dio.jenkins.plugins.artifact_manager_jclouds.s3.S3BlobStoreConfig.deleteArtifacts=true" value set.
If I delete a single build then the corresponding artifacts are deleted from S3. If I delete the whole project then none of the artifacts are deleted in S3.
Could be considered a core bug in that JCloudsArtifactManager.delete ought to have been called for all the builds, but that would be inefficient anyway.
This is also something that's impacting us. We're using the Bitbucket Branch Source plugin, and if any of our developers delete a branch from Bitbucket then Jenkins correctly removes the branch but any artifacts would remain in S3. Deleting a build through Jenkins does correctly remove the artifacts from S3, however.