Deprecate method that throws deprecation warning along the way.

src/View/Helper/NumberHelper.php

@@ -218,6 +218,7 @@ class NumberHelper extends Helper
      * if $currency argument is not provided. If boolean false is passed, it will clear the
      * currently stored value. Null reads the current default.
      * @return string|null Currency
+     * @deprecated 3.9.0 Use setDefaultCurrency()/getDefaultCurrency() instead.
      */
     public function defaultCurrency($currency)
     {