In Drupal 7 we could clear a cached URL with the following.
$url = url('<front>', array('absolute' => TRUE));
cache_clear_all($url, 'cache_page');
What's the equivalent in Drupal 8?
There is no equivalent in D8. What would be the closest to this is to assign a custom cache tag to the page and then invalidate it. See https://www.drupal.org/docs/8/api/cache-api/cache-tags.
But normally you don't need to invalidate cache tags in custom code. If you set the correct cache metadata this should happen automatically.