>A best-effort deletion could be beneficial for any node. It reduces storage requirements a bit.
As far as I'm aware, any node is free to delete its own data. In fact, isn't only storing data at the user's explicit request in the first place? It just can't do anything about what other nodes choose to keep or delete. If you're referring to a particular node wanting to delete do a best-effort deletion of data on other nodes, it's not clear to me why node A cares about reducing storage on nodes B, C and D (if you own those nodes, delete it yourself; if you don't.. then I don't know what you're up to)
A way to mark something as deleted, in a form of another ipfc object, could serve as soft delete, and also a permission to actually delete hunks of the object marked as deleted.
This, of course, is not secure deletion, and should not be.