When a request with some error status code is rendered, the servlet container (Tomcat/...) displays a default error page. You can configure custom error-pages in the web.xml file.

For the sake of a more dynamic configuration without the need to edit web.xml (and restart the instance), we can provide a default URI mapping for all error pages. This will enable users to create custom error pages with decoration.

I would propose to provide a default mapping on both 6.3 and 6.2, but change the web.xml only on 6.3. The 6.2 users can update their web.xml if they choose so.

See: https://docs.magnolia-cms.com/product-docs/6.2/administration/custom-error-page/