How to add a Custom Serializer to alter the Output of REST EXPORT
The plugin was missing the proper annotation (added now in the question). Views plugins use Annotation Discovery.

Adding something like the following solved the problem:

/**
 * My custom serializer
 *
 * @ViewsStyle(
 *   id = "hindi_custom_serializer",
 *   title = @Translation("Hindi Custom Serializer"),
 *   help = @Translation("Serializes views row data using the Serializer component."),
 *   display_types = {"data"}
 * )
 */
class HindiCustomSerializer extends Serializer {
  ....
}


Also worth noting that you don't need to include the implements CacheableDependencyInterface since is already declared by the \Drupal\rest\Plugin\views\style\Serializer plugin which you are extending from.