The TL;DR is that Amazon S3 now supports "conditional writes" which are guaranteed to fail if the file was written by some other writer. This is implemented by sending the ETag of an object's expected version alongside the write request.
Litestream now depends on this functionality to handle multiple writers. Think of optimistic locking.
Do you have a reference for this? I assume by CAS you mean content addressable storage? I googled but can't find any AWS docs on this.