> your idea of a deep copy and that author of that type's idea may not be the same

Definitely true that they may have done something weird, but I think the idea of a deep copy is pretty easy to define: It should not be possible to modify the original object using only the deep copy. (Note that this doesn't preclude copy-on-write, etc.)

I think it is quite weird that so few languages (even GC'd ones) provide a proper solution for this. It's clearly a thing people want to do.