I'm probably missing something, but what's the point of Special404? It seems to be about the same as modifying the core noarticletext message. It also requires modifying the web server configuration, so I'm not sure if it's appropriate for the Miraheze environment.
@DekuPH2006AJHalili I read that, but it doesn't explain why this is better than just modifying the mediawiki:noarticletext message, which is displayed when a page is not found. It seems that the extension requires setting up a separate 404 handler for the webserver, pointing to the extension's special page. But MediaWiki already has a built-in page that it displays in those situations.
You can find the name of the message to modify on any page with the ?uselang=qqx special language code. For example, if you want to change the message shown for invalid special page names, go to e.g. https://en.wikipedia.org/wiki/Special:BadPageName?uselang=qqx and see that the message is nospecialpagetext (the BadPageName is just a random string that is not a valid special page name; it could be anything).