How to index created field in baseFieldDefinition?
The way to do that at the moment is a custom storage schema handler, see NodeStorageSchema for example:

protected function getEntitySchema(ContentEntityTypeInterface $entity_type, $reset = FALSE) {
  $schema = parent::getEntitySchema($entity_type, $reset);

  $schema['node_field_data']['indexes'] += [
    'node__frontpage' =&gt; ['promote', 'status', 'sticky', 'created'],
    'node__title_type' =&gt; ['title', ['type', 4]],
  ];

  return $schema;
}


And then you register it with:

"storage_schema" = "Drupal\node\NodeStorageSchema",


as a handler.

To update existing installations, you need to drun drush entup or write an update function.