When a Content Library is deleted, what happens to the files contained on the backing storage?

Files are not left unchanged - vSphere actively removes content files from the backing storage when the library is deleted.

When an administrator deletes a Content Library, vSphere permanently removes both the library object and all of its content files from the backing datastore. This is a destructive operation, so administrators must ensure files are no longer needed or are backed up before deletion.

vSphere does not mark files as orphaned upon library deletion; the files themselves are removed along with the library object.

The containing folder is not marked as orphaned; the deletion process removes the library contents and cleans up the associated storage location.