#autocomplete_path is replaced by #autocomplete_route_name and #autocomplete_route_parameters for autocomplete fields

#autocomplete_path is removed and replaced with #autocomplete_route_name and #autocomplete_route_parameters

D7

  $form['name'] = array(
    '#type' => 'textfield',
    '#autocomplete_path' => 'user/autocomplete',
  );

D8

  $form['name'] = array(
    '#type' => 'textfield',
    '#autocomplete_route_name' => 'user.autocomplete',
  );

If values need to be appended to the end of the path, like the taxonomy term autocomplete, there is an #autocomplete_route_parameters property that takes an associative array of parameters. The keys must match the method parameters of the autocomplete controller.

D7

  $form['term'] = array(
    '#type' => 'textfield',
    '#autocomplete_path' => 'taxonomy/autocomplete/' . $field_name,
  );

D8

  $form['term'] = array(
    '#type' => 'textfield',
    '#autocomplete_route_name' => 'taxonomy.autocomplete',
    '#autocomplete_route_parameters' => array('field_name' => $field_name),
  );