// Invalide a Node cache.
\Drupal::service('cache_tags.invalidator')->invalidateTags(['node:5']);
// Invalidate a custom entity cache.
\Drupal::service('cache_tags.invalidator')->invalidateTags(['custom_entity:5']);
// Invalidate a custom cache tag.
\Drupal::service('cache_tags.invalidator')->invalidateTags(['thing:identifier']);