@piotr-cz can you make a PR?

I'm not sure where to store the map. Should it be JErrorPage?

BTW: The HTTP reason phrase for 403 is Dost%C4%99p zabroniony

Make a Response object that has these mappings. To save some work, just grab the data out of Symfony's Response object. Now there's a valid map for all known standard codes.

You could easily throw it in JResponse; undeprecate the class (it was deprecated since we moved all of its existing logic to the web application class) and add the new logic there.

Shouldn't the map go to JApplicationWeb then?
I just saw there is already a $responseMap array

That works too. I didn't know if it had a hard reference to the redirect code or if it was "good enough" for general use off the top of my head.

There are actually more instances with misc entries:

https://github.com/joomla/joomla-cms/blob/staging/administrator/components/com_redirect/models/fields/redirect.php#L38
https://github.com/joomla/joomla-cms/blob/staging/libraries/joomla/application/web.php#L84
https://github.com/joomla/joomla-cms/blob/staging/libraries/vendor/joomla/application/src/AbstractWebApplication.php#L78

I'd love to split whole phrases like HTTP/1.1 300 Multiple Choices in JApplicationWeb::$responseMap into protocol version, code and text just like in Symfony\Component\HttpFoundation\Response or Slim\Http\Response, make property public static and refactor JApplicationWeb::redirect method accordingly.

As ATM it's private, are there any reasons to not to do so?

Moreover what's the workflow?
Start with Joomla Framework Application AbstratWebApplication
then JApplicationWeb
and then adjust JDocumentError::render method?

btw @piotr-cz if you search whole code for HTTP/1.1 you will find more files in which they can be found than I posted earlier but I don't know if there is need for them to be translatable (even to plain ASCII form).

With e4d0451 the status header only includes the Exception message if error reporting is at maximum.