Validation plugin for a Boolean field
If I add $this->argument->argument = '1'; to the above code, it works:

&lt;?php

namespace Drupal\my_module_validation\Plugin\views\argument_validator;

use Drupal\views\Plugin\views\argument_validator\ArgumentValidatorPluginBase;

/**
 * Validate the New boolean.
 *
 * @ingroup views_argument_validate_plugins
 *
 * @ViewsArgumentValidator(
 *   id = "my_module_validation",
 *   title = @Translation("MYMODULE Validation")
 * )
 */
class MyModuleValidation extends ArgumentValidatorPluginBase {

  public function validateArgument($argument) {
    if($argument == 'new' || $argument == 'New' || $argument == '1') {
      $this-&gt;argument-&gt;argument = '1';
      return TRUE;
    }
    return FALSE;
  }
}