The first one I encountered was DrFTPD circa 2004. But these days, any object storage system qualifies because they all support varying replication schemes and reading from any valid in-sync replica.
In my book, a parallel filesystem is not just pooling together a bunch of nodes, but something that can actually support the synchronized accesses needed by a parallel workload. So not just decoupling between data and metadata, but scaling out of the metadata layer as well.
That and a hierarchical namespace (I could be sold on compromising some POSIX compliance for performance reasons, but it has to fundamentally be a hierarchical namespace with similar semantics). So object stores would not qualify.
in most filesystems files are addressed by an inode. and directories are just lists of inodes. if you remove those then you end up with a KV store: inode->file.
consequently i see no difficulty to convert a KV store into a hierarchy by adding the necessary tables/directories representing the hierarchy.