Drupal - How to get a parameter value from the current URL in views-view-fields.html.twig?

You could use Twig's split filter, though I don't recommend scripting too much inside templates.

Instead I recommend to pass the value over to Twig from a preprocess function from inside your theme or a custom module via template_preprocess_views_view_fields.

/**
 * Implements template_preprocess_views_view_fields().
 */
function MYTHEME/MYMODULE_preprocess_views_view_fields(&$variables) {

  // Retrieve an array which contains the path pieces.
  $current_path = \Drupal::service('path.current')->getPath();

  if ($current_path[0] == 'questions') {
    $variables['question_id'] = $current_path[1];
  }
}

And then inside your template print it for example like so:

{% if question_id %}
  <div id="{{ question_id }}">Question ID: {{ question_id }}</div>
{% endif %}

I don't know it is best way or not but can help you You are using the {% set url = url('current>') %} After dump, it turned out that's what

array(3) { 
         ["#markup"]=>
                 string(23) "http://example.com/category" ["#cache"]=> array(3) { 
      ["contexts"]=> array(2) { [0]=> string(5) "route" [1]=>string(8) 
            "url.site" } ["tags"]=> array(0) { } ["max-age"]=> int(-1) } 
        ["#attached"]=> array(0) { } }

Convert array to sting

{% set url = url('current>')['#markup'] %}

{{ url|split('/')|last }}

you will get the last value from theurl