SF_11.0, SF_11.1, SF_11.2, SF_12.0, SF_12.1, SF_12.2, SF_13.0, SF_13.1, SF_13.2, SF_13.3 - https://www.progress.com/documentation/sitefinity-cms/change-the-response-status-of-the-custom-error-page-of-mvc-pages-mvc

HandleHttpStatusCodeAttribute_Global.asax.cs

using System;
using Telerik.Sitefinity.Abstractions;

namespace SitefinityWebApp
{
    public class Global : System.Web.HttpApplication
    {
        protected void Application_Start(object sender, EventArgs e)
        {
            Bootstrapper.Bootstrapped += Bootstrapper_Bootstrapped;
        }

        protected void Bootstrapper_Bootstrapped(object sender, EventArgs e)
        {
            HandleHttpStatusCodeAttribute.Register();
        }
    }
}