Drupal 8 will actively complain when your site does not have a hash_salt configured, which usually gets generated when installing the site. (The complaint, mind you, might be fairly obscure; your site might just say "The website encountered an unexpected error. Please try again later." Depending on your error reporting settings, the message might be a bit more helpful). If, for example, you "install" a site by copying over a database and files, you will not have this.
Update 26/21/2021: since writing this blog post, I found that basically, for Drupal's functioning, it does not matter what your hash salt is. You can put in any old string, which would be fine especially for a development system. The procedure below gets you something that is similar to what the official Drupal install procedure would produce.