CustomPlugin.php

<?php

declare (strict_types=1);
namespace com\example\PluginNamespace;

use Drupal\Component\Plugin\Attribute\Plugin;

/**
 * Custom plugin attribute.
 */
class CustomPlugin extends Plugin {
    
    /**
     * Constructs a CustomPlugin attribute object.
     *
     * @param string $id
     *   The attribute class ID.
     * @param string $title
     *   The title.
     */
    public function __construct(string $id, string $title) {
    }

}

/**
 * Custom plugin attribute.
 */
class CustomPlugin2 extends Plugin {

}