includes/managers/class-fs-admin-menu-manager.php
static function add_subpage(
			$parent_slug,
			$page_title,
			$menu_title,
			$capability,
			$menu_slug,
			$function = ''
		) {
			$fn = 'add_submenu' . '_page';

			return $fn( $parent_slug,
				$page_title,
				$menu_title,
				$capability,
				$menu_slug,
				$function
			);
		}
static function add_subpage(
			$parent_slug,
			$page_title,
			$menu_title,
			$capability,
			$menu_slug,
			$function = ''
		) {
			$fn = 'add_submenu' . '_page';

			return $fn( $parent_slug,
				$page_title,
				$menu_title,
				$capability,
				$menu_slug,
				$function
			);
		}
static function add_subpage ( $ parent_slug , $ page_title , $ menu_title , $ capability , $ menu_slug , $ function = '' ) { $ fn = 'add_submenu' . '_page' ; return $ fn ( $ parent_slug , $ page_title , $ menu_title , $ capability , $ menu_slug , $ function ) ; }
Add a submenu page.

Note for WordPress.org Theme/Plugin reviewer:

This is a replication of `add_submenu_page()` to avoid Theme Check warning.

Why?
====
Freemius is an SDK for plugin and theme developers. Since the core
of the SDK is relevant both for plugins and themes, for obvious reasons,
we only develop and maintain one code base.

This method will not run for wp.org themes (only plugins) since theme
admin settings/options are now only allowed in the customizer.

If you have any questions or need clarifications, please don't hesitate
pinging me on slack, my username is @svovaf.

@author Vova Feldman (@svovaf)
@since  1.2.2

@param string          $parent_slug The slug name for the parent menu (or the file name of a standard
WordPress admin page).
@param string          $page_title  The text to be displayed in the title tags of the page when the menu is
selected.
@param string          $menu_title  The text to be used for the menu.
@param string          $capability  The capability required for this menu to be displayed to the user.
@param string          $menu_slug   The slug name to refer to this menu by (should be unique for this menu).
@param callable|string $function    The function to be called to output the content for this page.

@return false|string The resulting page's hook_suffix, or false if the user does not have the capability
required.
Add a submenu page .