Responses are not cacheable

Looks good to me! Thanks.

This is a bad idea, 404 responses shouldn't be cached (for many reasons).

The two biggest reasons are that anyone can then easily overwhelm your cache by filling it with nonsense, and also - say you want to add something to your site, but it's already in the cache as a 404 - it'll carry on 404ing even though it exists!

Drupal supports caching 4xx responses, so we can make it depend on the setting:

/**
 * Cache TTL for client error (4xx) responses.
 *
 * Items cached per-URL tend to result in a large number of cache items, and
 * this can be problematic on 404 pages which by their nature are unbounded. A
 * fixed TTL can be set for these items, defaulting to one hour, so that cache
 * backends which do not support LRU can purge older entries. To disable caching
 * of client error responses set the value to 0. Currently applies only to
 * page_cache module.
 */
# $settings['cache_ttl_4xx'] = 3600;