Simple filter for adding a menu separator at a specific location on the WordPress admin menu.

wp-menu-separator.php

<?php
/**
 * Adds a separator to the administration menus in a specific location.
 *
 * @param array $menu Associative array of administration menu items.
 *
 * @return array $menu Modified associative array of administration menu items.
 */
add_filter( 'add_menu_classes', function( $menu ) {
	$menu['44'] = array(
		null,
		'edit_posts',
		'separator-custom',
		null,
		'wp-menu-separator',
	);

	ksort( $menu );

	return $menu;
} );