By default, the label for the search form text input in Drupal 8 is set to invisible, that is, it's present but hidden by applying the "visually-hidden" class so users of assistive technology can still read it. But what if you want to display it visually?
That's not so easy. There's no way to do it through configuration, and little to no information about how to do it with code.
I muddled around and finally found where the visibility was being set, in SearchBlockForm.php in the core Search module. In there we find: