Same as Update paginate_links() with 4 new arguments - #3 by james
This should be added as a new value accepted for the existing $args array that this function receives, rather than a new parameter to the wp_dropdown_categories function.
The reason is that if we add new parameters to functions, and then WP ends up also adding a different new parameter to the same function, we have a mess and no way to retain compatibility. When there is another way to achieve something like this without changing a function signature it is best to do it that way. The $args arrays are perfect for this.
As long as it’s done that way, then no problem here.