Yes, you are absolutely right. "EBS volumes can only be increased, not decreased".
I'm quite happy to lose data, or manage my data's physical location on disk, and do online decreases… but I can't!
(Yes I know that's probably not what you meant, but it does highlight the question of what exactly is non-valuable data?)
> what exactly is non-valuable data?
Caches, mirrors, backing volumes for redundant data stores or processing infrastructure that indicates to try again on another node on failure.