Is there something equivalent to template_preprocess_search_results()? I just realized that template_preprocess_search_results() function has been left out in Drupal 8 for some reasons, although template_preprocess_search_result() (for single results) is still there.

I need to be able to manipulate the search results prior to rendering as I would with template_preprocess_search_results() in Drupal 7.
You can create a new module and inside your YOUR_MODULE.module file put this code:

function YOUR_MODULE_preprocess_item_list__search_results(&amp;$variables) {
  //to use dpm() you nedd the devel module
  dpm($variables['items']);
}


Install your module and you will see the output in the search result