Group WooCommerce downloads by product in a dropdown menu

You need to check first that $download['list'] exist, to avoid that issue.

To get a dropdown of downloads grouped by product, some changes and additions are needed (JavaScript/jQuery is required).

Note (update): On email notifications, we keep the Downloads table as it is by default, as the dropdown can’t work.

The following code will handle this dropdown everywhere, in the front end:

// Group downloads data by product
add_filter( 'woocommerce_customer_get_downloadable_products', 'prefix_group_downloadable_products', 10, 2 );
add_filter( 'woocommerce_order_get_downloadable_items', 'prefix_group_downloadable_products', 10, 2 );
function prefix_group_downloadable_products( $downloads = array(), $order = null ) {
    // Only on front-end
    if ( is_admin() || ! is_wc_endpoint_url() ) { 
        return $downloads;
    }

    $unique_downloads = []; // Initializing

    foreach ( $downloads as $download ) {
        $list = [
            'download_url' => $download['download_url'],
            'file_name'    => $download['file']['name']
        ];

        if ( array_key_exists( $download['product_id'], $unique_downloads ) ) {
            $unique_downloads[ $download['product_id'] ]['list'][] = $list;
            continue;
        }

        $data = $download;
        $data['list'] = [ $list ];
        $unique_downloads[ $download['product_id'] ] = $data;
    }
    return $unique_downloads;
}

// Display a dropdown of the downloads by product
add_action( 'woocommerce_account_downloads_column_download-file', 'customize_downloads_columns' );
function customize_downloads_columns( $download = array() ) {
    $lists = isset($download['list']) ? $download['list'] : array();

    if ( empty( $lists ) ) {
        _e( 'No Download Files', 'storefront' );
        return;
    }

    echo '<select class="downloads-dropdown">
    <option value="">'. __(' Select download', 'storefront' ) .'</option>';

    foreach ( $lists as $list ) {
        printf( '<option value="%s">%s</option>', 
        esc_url($list['download_url']), esc_html($list['file_name']) );

    }
    echo '</select>';
}

// Javascript: Trigger the download when selecting a file in the dropdown
add_action( 'wp_head', 'trigger_download_from_selected_file_js' );
function trigger_download_from_selected_file_js() {
    if ( is_wc_endpoint_url('downloads') || is_wc_endpoint_url('view-order') || is_wc_endpoint_url('order-received') ) {
        wc_enqueue_js("$(document.body).on('change', 'select.downloads-dropdown', function(){
        if( $(this).val() != '' ) {
            window.location.href = $(this).val();
        }
        });");
    }
}

You will get something like (in My account > Downloads page):

In My account > View Order and in Order received (thankyou) pages:

The downloads table on email notifications stays unchanged (default WooCommerce behavior):

Addition for email notifications (optional)

Replace the downloads table, with a text linked to My Account "downloads" section:

// Remove the downloads table
add_action( 'woocommerce_init', function(){
    remove_action( 'woocommerce_email_order_details', array( WC()->mailer(), 'order_downloads' ), 10 );
});

// Email notifications: Display a text linked to My Account downloads
add_action( 'woocommerce_email_order_details', 'display_linked_text_to_my_account_downloads', 9, 4 );
function display_linked_text_to_my_account_downloads( $order, $sent_to_admin = false, $plain_text = false, $email = '' ) {
    $show_downloads = $order->has_downloadable_item() && $order->is_download_permitted() && ! $sent_to_admin && ! is_a( $email, 'WC_Email_Customer_Refunded_Order' );
    
    if ( ! $show_downloads ) {
        return;
    }

    $text_align    = is_rtl() ? 'right' : 'left';
    $downloads_url = wc_get_endpoint_url('downloads', '', get_permalink( get_option('woocommerce_myaccount_page_id') ) );
    ?>
    <h2 class="woocommerce-order-downloads__title"><?php esc_html_e( 'Downloads', 'woocommerce' ); ?></h2>
    <table class="td" cellspacing="0" cellpadding="6" style="width: 100%; font-family: 'Helvetica Neue', Helvetica, Roboto, Arial, sans-serif; margin-bottom: 40px;" border="1">
        <tr>
            <td class="td" style="text-align:<?php echo esc_attr( $text_align ); ?>;">
                <?php printf( __('Downloads are available in your Account %s.', 'storefront'),
                '<a href="' . $downloads_url . '" class="button">'. __('"Downloads" section', 'storefront') .'</a>'); ?>
            </td>
        </tr>
    </table>
    <?php 
}