Support an independent max-age for 4xx responses

     $is_cacheable = ($this->requestPolicy->check($request) === RequestPolicyInterface::ALLOW) && ($this->responsePolicy->check($response, $request) !== ResponsePolicyInterface::DENY);
+
     $client_err

1. Seems out of scope

     $max_age = $response->isClientError() && isset($client_error_max_age)
       ? $client_error_max_age
         : $this->config->get('cache.page.max_age');
+
     // Add headers 

2. Same

3. 3 more instances of spacing being added which is out of scope

and patch #129 is removing test coverage.

Using patch #124 on D 10.1.8 fixed my issue.

@smustgrave regarding extra lines in interdiff 124-128 (point 1., 2.) - an empty line was removed in 124, so these are OK in #128 I think.
Point 3. and test coverage should be fixed.

I think the assignment is not relevant after more than 1 year.