When you embed the relevant (not necessarily that of object creation) timestamp as a prefix, it sure becomes one. Whether that prefix is part of the "path" (object/path/prefix/with/<4-digit year/)" or directly part of the basename (object/path/prefix/to/app-specific/files/<4-digit year>-<2-digit month>-....), being able to limit the search space server-side becomes incredibly useful.
You can try it yourself: list objects in a bucket prefix with lots of files, and measure the time it takes to list all of them vs. the time it takes to list only a subset of them that share a common prefix.
I think 99% of S3 usage just consists of retrieving objects with known keys. It seems odd to me to consider prefix listing as a key feature.