Previously, menus could be registered with an integer slug. This used to cause unintended issues that are difficult to debug. Consider the following use of register_nav_menus: The assumption is that the resulting list of menus would match what was passed. Instead, the result is this: This would cause wp_nav_menu( array( ‘theme_location’ => 1 ) ) … Continue reading Integer menu slugs are no longer supported from WordPress 5.3