The message is informational as noted by the text - it's to inform the viewer that garbage reclamation (of space no longer needed by deleted backups) has proceeded to clear the old data from storage.
clear old data from "storage", I assume the "storage" here means local cache. Correct? Any way can I distinguash "old data" was in local cache or Cloud?
Storage refers to either local cache and/or cloud. And no, you can't distinguish the effects other than the amount of space that is reclaimed when you look at the storage optimization graph. The downward curves reflect when GC and reclamation have taken place to clear space.
