As lazy we humans are we tent to forget the Revision log message and move on but that us bad for our system and it is a good habit to save data about changes you have done so this is how you can Set Revision Log Message Programmatically
use Drupal\node\NodeInterface;
use Drupal\Core\Entity\EntityInterface;
/**
* Implements hook_entity_presave().
*/
function custom_module_entity_presave(EntityInterface $entity) {
switch ($entity->bundle()) {
case 'page_index':
if ($entity instanceof NodeInterface) {
$message = '';
$entity->revision_log->value = $message;
}
break;
}
}