'clean_url' configuration setting removed; clean URLs are autodetected each request

In Drupal 7, there was a 'clean_url' configuration variable, and a admin/config/search/clean-urls form for configuring it. This has been removed from Drupal 8. Instead, if you access a Drupal page whose path is 'some/path' as http://example.com/some/path, then Drupal detects this as a clean URL and outputs all links from that page as clean URLs. On the other hand, if your site is on a server where clean URLs do not work, you can access that page as http://example.com/index.php/some/path, and when you do, Drupal outputs all links from that page as "dirty" URLs (i.e., with the "index.php" included).

If you have publicly exposed URLs that include old-style dirty URLs, using ?q=, it is recommended to adjust your URLs to the new format ASAP. To provide automatic forwarding for ?q=-style URLs to avoid SEO breaks, install the Legacy Redirect module.

There is some in-progress work for handling upgrade issues and edge cases in #1546082: Follow-up to variable_get('clean_url') removal.

As a result of this change, modules that display label prefixes in front of form input elements containing Drupal paths can be simplified as follows.

Drupal 7:

function path_admin_form($form, &$form_state, ...) {
  $form['source'] = array(
    '#type' => 'textfield', 
    '#title' => t('Existing system path'), 
    '#description' => t('Specify the existing path you wish to alias. For example: node/28, forum/1, taxonomy/term/1.'), 
    '#field_prefix' => url(NULL, array('absolute' => TRUE)) . (variable_get('clean_url', 0) ? '' : '?q='), 
  );
  ...
}

Drupal 8:

function path_admin_form($form, &$form_state, ...) {
  $form['source'] = array(
    '#type' => 'textfield', 
    '#title' => t('Existing system path'), 
    '#description' => t('Specify the existing path you wish to alias. For example: node/28, forum/1, taxonomy/term/1.'), 
    '#field_prefix' => url(NULL, array('absolute' => TRUE)),
  );
  ...
}