I'm not sure of a name, but Tiddlywiki does this. You create hierarchies by having tags by creating an item with the tag as its name which causes all items with said tag to be a child of that item. This has a nice side effect of allowing items to exist in multiple locations (so no unique parent is enforced) while still requiring the graph to be acyclic.

It ends up working kind of like hard links for folders/files, but it is a lot easier to setup since child items are the ones which declare where they are located, not the parents/directories. I think another reason why hard links are more difficult to use than this particular system is that with Tiddlywiki, it is easy to see all the locations an item falls under at once as well as seeing all the items at a particular location. I feel like adding this reverse location information would be quite helpful and would be less of a change than implementing tags for existing filesystems.