Exposed Views Filter of Usernames in a list instead of autocomplete

If you need a simple approach, you can install the drupal Chosen module, which does the heavy lifting.

With coding, you can use hook_form_FORM_ID_alter() as the following:

use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_FORM_ID_alter().
 */
function MODULE_form_views_exposed_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  // Check the views form ID.
  if ($form['#id'] != 'YOUR-VIEWS-FORM-ID') {
    return FALSE;
  }

  // Define the form options array.
  $options = [];

  // Load all user objects.
  $users = \Drupal::entityTypeManager()
    ->getStorage('user')
    ->loadMultiple();

  foreach ($users as $user) {

    // You can add filtration here e.g.
    if ($user->hasRole('anonymous')) {
      continue;
    }
    $options[$user->id()] = $user->getDisplayName();

    $form['YOUR_FILED']['#type'] = 'select';
    $form['YOUR_FILED']['#multiple'] = FALSE;

    // Specify the empty option for our select list
    $form['YOUR_FILED']['#empty_option'] = t('Select user');

    // Add the $options from above to our select list
    $form['YOUR_FILED']['#options'] = $options;
    unset($form['YOUR_FILED']['#size']);
  }

}