Delete all content entities in Drupal 8.

delete_content_entities.d8.php

<?php

// Delete all nodes.
entity_delete_multiple('node', \Drupal::entityQuery('node')->execute());

// Delete all files.
entity_delete_multiple('file', \Drupal::entityQuery('file')->execute());

// Delete all taxonomy terms.
entity_delete_multiple('taxonomy_term', \Drupal::entityQuery('taxonomy_term')->execute());

// Delete all block content.
entity_delete_multiple('block_content', \Drupal::entityQuery('block_content')->execute());

// Delete all menu links.
entity_delete_multiple('menu_link_content', \Drupal::entityQuery('menu_link_content')->execute());

// Delete all users except 1 and 0.
entity_delete_multiple('user', \Drupal::entityQuery('user')->condition('uid', '1', '!=')->condition('uid', '0', '!=')->execute());