I don't think this is necessarily true. You could just have malloc() bump an allocation pointer by the size, return the pointer pre-bump, or return NULL if we run out of space. Then free() can do nothing.

If you decide you want to have a malloc/free implementation that isn't as garbage as that, then you need some way to ensure malloc() does not ever return a pointer to memory that was already handed out - and I would argue that the book-keeping for this is this metadata being talked about. Even if free() took a "size" parameter, this would still be needed.