Add dropdown classes to your wp_nav_menu() output

bootstrap-dropdown-wp-nav-menu.php

<?php
/**
 * Plugin Name:       Bootstrap Dropdown on WP Nav Menu
 * Description:       Add dropdown classes to your wp_nav_menu() output
 * Version:           1.0.0
 * Author:            Mário Valney
 * Author URI:        https://mariovalney.com
 * Text Domain:       bootstrap-dropdown-wp-nav-menu
 */
 
function boostrap_menu_nav_menu_submenu_css_class( $classes, $args, $depth ) {
    if ( empty( $depth ) ) {
        $classes[] = 'dropdown-menu';
    }

    return $classes;
}

function boostrap_menu_nav_menu_link_attributes( $atts, $item, $args, $depth ) {
    if ( ! empty( $depth ) && in_array( 'menu-item-has-children', ( $item->classes ?? [] ) ) ) {
        $atts['data-toggle'] = 'dropdown';
    }

    return $atts;
}

function boostrap_menu_wp_nav_menu_objects( $sorted_menu_items, $args ) {
    foreach ( $sorted_menu_items as $menu_item ) {
        if ( ! in_array( 'menu-item-has-children', ( $menu_item->classes ?? [] ) ) ) {
            continue;
        }

        if ( ! empty( $menu_item->menu_item_parent ) ) {
            continue;
        }

        $menu_item->classes[] = 'dropdown';
    }

    return $sorted_menu_items;
}

add_filter( 'nav_menu_submenu_css_class', 'boostrap_menu_nav_menu_submenu_css_class', 10, 3 );
add_filter( 'nav_menu_link_attributes', 'boostrap_menu_nav_menu_link_attributes', 10, 4 );
add_filter( 'wp_nav_menu_objects', 'boostrap_menu_wp_nav_menu_objects', 10, 2 );