Create your custom 404 page in Notion (remember that the page must be included in your site’s structure).
Add the page to your site in the Super dashboard:
Click on the three-dots button next to your page:
Choose “Use as custom page”:
Done - the page now shows a notice that it’s being used for 404 status codes on the site:
If a visitor clicks through to a page that no longer exists, they won’t see the default 404 page - they’ll see your custom one instead.