S3 is object storage and volumes need a full file system, making then comparable with the EFS in the lingo of AWS services, not S3.
That said, by configuring Discourse per Configure an S3 compatible object storage provider for uploads and Configure Discourse to use a separate PostgreSQL server you get a mostly stateless Discourse. Hook it behind AWS ALB for TLS termination and AWS Elasticache for Redis and you are full stateless.