You are right. Based on this page S3 is supported. My file with this function:

from clearml import Task

task = Task.init(project_name='s3_upload_models', task_name='sklearn', output_uri=True)
task.update_output_model(auto_delete_file=False, name='v0.0.1', model_path='s3://<BUCKET_NAME>/MLOps/models/sklearn/sklearn.pkl')
task.close()

my clearml.conf

aws {
    s3 {
        # default, used for any bucket not specified below
        key: ""
        secret: ""
        region: ""

        credentials: [
            {
                # This will apply to all buckets in this host (unless key/value is specifically provided for a given bucket)
                host: "<s3host>:443"
                key: "<ACCESS_KEY>"
                secret: "<SECRET_KEY>"
                multipart: false
                secure: false
                verify: false # OR "/path/to/ca/bundle.crt" OR "

" OR false to not verify
            }
        ]
    }


    boto3 {
        pool_connections: 512
        max_multipart_concurrency: 16
        multipart_threshold: 8388608 # 8MB
        multipart_chunksize: 8388608 # 8MB
    }
}