What is deprecated code?

Deprecating code is a way of identifying code that will be removed in a future major version.

For example, the drupal_set_message() function was deprecated in Drupal 8.5 and removed in Drupal 9 as the messenger service replaced it.

Once it was deprecated, the function was changed to use the new service to avoid duplicating code and a message was added to notify Developers:

function drupal_set_message($message = NULL, $type = 'status', $repeat = FALSE) {
@trigger_error('drupal_set_message() is deprecated in Drupal 8.5.0 and will be removed before Drupal 9.0.0. Use \Drupal\Core\Messenger\MessengerInterface::addMessage() instead. See https://www.drupal.org/node/2774931', E_USER_DEPRECATED);

$messenger = \Drupal::messenger();
if (isset($message)) {
$messenger->addMessage($message, $type, $repeat);
}

return $messenger->all();
}

This approach means that code can be refactored without breaking backwards-compatibility and, to upgrade any custom code to be compatible with Drupal 9, any references to drupal_set_message() just needed to be updated to use the new Messenger service.

No large rewrite needed!