The multisite_over_quota_message() WordPress PHP function displays the out of storage quota message in a Multisite environment.
Usage
To use the multisite_over_quota_message() function, simply call it in the appropriate location within your theme or plugin:
multisite_over_quota_message();
Parameters
- None
 
More information
See WordPress Developer Resources: multisite_over_quota_message
The multisite_over_quota_message() function is part of the WordPress core and is specifically designed for use in Multisite installations.
Examples
Display out of quota message on a custom admin page
If you are creating a custom admin page, you can use the multisite_over_quota_message() function to display the out of storage quota message if needed.
// In your custom admin page
function my_custom_admin_page() {
    // Display out of storage quota message if applicable
    multisite_over_quota_message();
    // Your admin page content here
}
Display out of quota message on a custom template
If you want to display the out of storage quota message on a specific template in your theme, you can use the multisite_over_quota_message() function.
// In your custom template get_header(); multisite_over_quota_message(); // Your template content here get_footer();
Display out of quota message on a shortcode
You can create a shortcode that displays the out of storage quota message using the multisite_over_quota_message() function.
// Create a shortcode to display the out of storage quota message
function my_quota_message_shortcode() {
    ob_start();
    multisite_over_quota_message();
    return ob_get_clean();
}
add_shortcode('my_quota_message', 'my_quota_message_shortcode');
Display out of quota message using a Gutenberg block
Create a custom Gutenberg block that displays the out of storage quota message using the multisite_over_quota_message() function.
// Register the custom Gutenberg block
function my_quota_message_block() {
    wp_register_script(
        'my-quota-message-block',
        get_template_directory_uri() . '/my-quota-message-block.js',
        array( 'wp-blocks', 'wp-element', 'wp-editor' )
    );
    register_block_type( 'my-theme/my-quota-message', array(
        'editor_script' => 'my-quota-message-block',
        'render_callback' => 'my_quota_message_block_callback',
    ));
}
add_action( 'init', 'my_quota_message_block' );
// Block callback function
function my_quota_message_block_callback() {
    ob_start();
    multisite_over_quota_message();
    return ob_get_clean();
}
Display out of quota message using a widget
Create a custom widget that displays the out of storage quota message using the multisite_over_quota_message() function.
class My_Quota_Message_Widget extends WP_Widget {
    public function __construct() {
        parent::__construct(
            'my_quota_message_widget',
            'My Quota Message Widget',
            array( 'description' => 'A widget to display the out of storage quota message.' )
        );
    }
    public function widget( $args, $instance ) {
        echo $args['before_widget'];
        multisite_over_quota_message();
        echo $args['after_widget'];
    }
}
add_action( 'widgets_init', function() {
    register_widget( 'My_Quota_Message_Widget'