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.cs

using System.Web;
using System.Web.Mvc;
using Telerik.Sitefinity.Services;
using Telerik.Sitefinity.Web.Events;

namespace SitefinityWebApp
{
    public class HandleHttpStatusCodeAttribute : ActionFilterAttribute
    {
        /// <summary>
        /// Called by the ASP.NET MVC framework after the action method executes.
        /// </summary>
        /// <param name="filterContext">The filter context.</param>
        public override void OnActionExecuted(ActionExecutedContext filterContext)
        {
            if (!HttpContext.Current.Items.Contains(HandleHttpStatusCodeAttribute.ChangeResponseStatusItemsKey))
            {
                if (SystemManager.IsDesignMode)
                {
                    return;
                }
                var item = new ChangeResponseStatusProxy()
                {
                    Status = this.Status,
                    StatusCode = this.StatusCode
                };
                HttpContext.Current.Items.Add(HandleHttpStatusCodeAttribute.ChangeResponseStatusItemsKey, item);
            }
        }

        public int StatusCode { get; set; }
        public string Status { get; set; }
        internal static void Register()
        {
            EventHub.Subscribe<IPagePreRenderCompleteEvent>(OnPagePreRenderCompleteEventHandler);
        }

        private static void OnPagePreRenderCompleteEventHandler(IPagePreRenderCompleteEvent @event)
        {
            if (HttpContext.Current.Items.Contains(HandleHttpStatusCodeAttribute.ChangeResponseStatusItemsKey))
            {
                var proxy = (ChangeResponseStatusProxy)HttpContext.Current.Items[HandleHttpStatusCodeAttribute.ChangeResponseStatusItemsKey];
                HttpContext.Current.Response.StatusCode = proxy.StatusCode;
                HttpContext.Current.Response.Status = proxy.Status;
            }
        }

        public const string ChangeResponseStatusItemsKey = "ChangeResponseStatus";
        public const string StatusItemsKey = "ChangeResponseStatus";
        public const string StatusCodeItemsKey = "ChangeResponseStatus";
    }
    internal class ChangeResponseStatusProxy
    {
        public int StatusCode { get; set; }

        public string Status { get; set; }
    }
}