Allow modules to alter messages during Node save

This is a great idea! It would be awesome to be able to alter the text at some point, and hope it's ok that I reopen the issue?

As a workaround until (or if) this lands, here's one method to tweak the message, from a MR for Simplenews:

use Drupal\Core\Entity\EntityForm;

/**
 * Implements hook_form_alter().
 *
 * Adds link to preview and send page above a newsletter.
 */
function simplenews_form_alter(&$form, &$form_state, $form_id) {
  $forms = [
    'node_simplenews_issue_form',
    'node_simplenews_issue_edit_form',
  ];
  $admin_or_send_rights = (\Drupal::currentUser()->hasPermission('administer newsletters') || \Drupal::currentUser()->hasPermission('send newsletter'));
  if (in_array($form_id, $forms) && $admin_or_send_rights) {
    // Adding custom callback to customize status message.
    $form['actions']['submit']['#submit'][] = '_post_node_save_callback';
  }
}

/**
 * Custom callback to customize status message.
 */
function _post_node_save_callback($form, &$form_state) {
  $form_object = $form_state->getFormObject();
  if ($form_object instanceof EntityForm) {
    $node = $form_object->getEntity();
    $node_id = $node->id();
    \Drupal::messenger()->addStatus(t('Preview or send it via the <a href="/node/' . $node_id .'/simplenews">Newsletter</a> tab, administer under <a href="/admin/content/simplenews">Newsletter issues</a>.'));
  }
}

Based on the great blog post Customize/override the “node has been created/updated” status message on Drupal 9 by Barbara Bombachini → .