void* is basically used for ad-hoc polymorphism in C, and it is a vital part of C programming.

    void new_thread(void (*run)(void*), void* context);

^- This let's us pass arbitrary starting data to a new thread.

I don't know whether this counts as "very few use cases".

The Memory Ownership advice is maybe good, but why are you allocating in the copy routine if the caller is responsible for freeing it, anyway? This dependency on the global allocator creates an unnecessarily inflexible program design. I also don't get how the caller is supposed to know how to free the memory. What if the data structure is more complex, such as a binary tree?

It's preferable to have the caller allocate the memory.

    void insert(BinTree *tree, int key, BinTreeNode *node);

^- this is preferable to the variant where it takes the value as the third parameter. Of course, an intrusive variant is probably the best.

If you need to allocate for your own needs, then allow the user to pass in an allocator pointer (I guessed on function pointer syntax):

    struct allocator { void* (*new)(size_t size, size_t alignment); void (*free)(void* p, size_t size); void* context; }.*