Create OptGroups for select dropdown (categories) in WordPress using selectize plugin

functions.php

<?php

class Top_level_Optgroup extends Walker_CategoryDropdown {

  var $optgroup = false;

  function start_el( &$output, $category, $depth = 0, $args = array(), $id = 0 ) {

    $pad = str_repeat('&nbsp;', $depth * 3);

    $cat_name = apply_filters('list_cats', $category->name, $category);
    if (0 == $depth) {
//    $this->optgroup = true;
      $output .= "<optgroup class=\"level-$depth\" label=\"".$cat_name."\" >";
    } else {
      $output .= "<option class=\"level-$depth\" value=\"".$category->term_id."\"";
      if ( $category->term_id == $args['selected'] )
              $output .= ' selected="selected"';
      $output .= '>';
      $output .= $pad.$cat_name;
      if ( $args['show_count'] )
              $output .= '&nbsp;&nbsp;('. $category->count .')';
      $output .= "</option>";
    }

  }

  function end_el(&$output,$object,$depth=0, $args = []) {
     if (0 == $depth/* && true == $this->optgroup*/) {
	 $output .= '</optgroup>';
      }
  }
}

// usage test
wp_dropdown_categories(
  array(
    'orderby' => 'name',
    'hide_empty' => 0,
    'show_count' => 1,
    'show_option_none' => 'Select one',
    'class' => 'cat',
    'hierarchical' => 1,
    'name' => 'cat',
    'walker' => new Top_level_Optgroup
  )
);
?>

Another little improvment..
$output .= "<option class=\"level-$depth\" value=\"".$category->term_id."\"";
with this code we assume that the value should be always the category ID, but in some cases we would want the option value to be what ever we put un the argument "value_field" passed to wp_dropdown_categories()
so, it would be better to set it dynamically, according to user $args put to the function, like this:

$output .= "<option class=\"level-$depth\" value=\"".$category->{$args['value_field']}."\"";
         
if ( $category->{$args['value_field']} == $args['selected'] ) {
     $output .= ' selected="selected"';
}

and again thanks @dsebao for the useful snippet
hope this helps some one