Working on a module in which I'm trying to utilize this hook. According to the API docs, the arguments to this hook are:
\Drupal\views\ViewExecutable $view: The view object about to be processed.
string $output: A flat string with the rendered output of the view.
\Drupal\views\Plugin\views\cache\CachePluginBase $cache: The cache settings.
However, the second argument ($output), does not in fact, come in as a "flat string". It comes in as an array. Also, shouldn't that be "&$output", instead of "$output"?