An Umbraco 8 component/composer for protecting pages that are configured as 404 pages in umbracoSettings.config.

ProtectConfiguredError404PagesComponent.cs

namespace MyApplication.Components
{
    using System.Collections.Generic;
    using System.Linq;

    using Umbraco.Core.Composing;
    using Umbraco.Core.Configuration.UmbracoSettings;
    using Umbraco.Core.Events;
    using Umbraco.Core.Models.Entities;
    using Umbraco.Core.Services.Implement;

    /// <summary>
    /// A component to protect content which is configured as an error404 page in the umbracoSettings.config from being unpublished or deleted.
    /// </summary>
    public class ProtectConfiguredError404PagesComponent : IComponent
    {
        /// <summary>
        /// The content error pages.
        /// </summary>
        private readonly IEnumerable<IContentErrorPage> contentErrorPages;

        /// <summary>
        /// Initializes a new instance of the <see cref="ProtectConfiguredError404PagesComponent"/> class.
        /// </summary>
        /// <param name="umbracoSettings">The umbraco settings.</param>
        public ProtectConfiguredError404PagesComponent(IUmbracoSettingsSection umbracoSettings)
        {
            this.contentErrorPages = umbracoSettings.Content.Error404Collection.ToArray();
        }

        /// <inheritdoc />
        public void Initialize()
        {
            if (this.contentErrorPages.Any())
            {
                ContentService.Deleting += this.ContentServiceOnDeleting;
                ContentService.Unpublishing += this.ContentServiceOnUnpublishing;
            }
        }

        /// <inheritdoc />
        public void Terminate()
        {
            if (this.contentErrorPages.Any())
            {
                ContentService.Deleting += this.ContentServiceOnDeleting;
                ContentService.Unpublishing -= this.ContentServiceOnUnpublishing;
            }
        }

        /// <summary>
        /// Creates an <see cref="EventMessage"/> for a given verb.
        /// </summary>
        /// <param name="verb">The verb causing the event message.</param>
        /// <returns>A <see cref="EventMessage"/> for the given verb.</returns>
        private EventMessage CreateMessage(string verb)
        {
            // category & message could be localized instead of hardcoded.
            return new EventMessage("404 Page Protection", $"Unable to {verb} page. This page is used by the Umbraco 404 handler.", EventMessageType.Error);
        }

        /// <summary>
        /// An event that triggers when <see cref="ContentService"/> Deleting event fires.
        /// </summary>
        /// <param name="sender">The sender.</param>
        /// <param name="e">The event args.</param>
        private void ContentServiceOnDeleting(Umbraco.Core.Services.IContentService sender, DeleteEventArgs<Umbraco.Core.Models.IContent> e)
        {
            if (this.ShouldCancel(e.DeletedEntities))
            {
                e.Cancel = true;
                e.Messages.Add(this.CreateMessage("delete"));
            }
        }

        /// <summary>
        /// An event that triggers when <see cref="ContentService"/> Unpublishing event fires.
        /// </summary>
        /// <param name="sender">The sender.</param>
        /// <param name="e">The event args.</param>
        private void ContentServiceOnUnpublishing(Umbraco.Core.Services.IContentService sender, PublishEventArgs<Umbraco.Core.Models.IContent> e)
        {
            if (this.ShouldCancel(e.PublishedEntities))
            {
                e.Cancel = true;
                e.Messages.Add(this.CreateMessage("unpublish"));
            }
        }

        /// <summary>
        /// Checks if the incoming collection of entities has a node which matches a error 404 page.
        /// </summary>
        /// <param name="entities">The entities.</param>
        /// <returns>A <see cref="bool"/> of whether it should be canceled or not.</returns>
        private bool ShouldCancel(IEnumerable<IEntity> entities)
        {
            foreach (var entity in entities)
            {
                if (this.contentErrorPages.Any(x => (x.HasContentId && x.ContentId == entity.Id) || (x.HasContentKey && x.ContentKey == entity.Key)))
                {
                    return true;
                }
            }

            return false;
        }
    }
}

ProtectConfiguredError404PagesComponentComposer.cs

namespace MyApplication.Composers
{
    using Umbraco.Core;
    using Umbraco.Core.Composing;

    using MyApplication.Components;

    /// <summary>
    /// A <see cref="IComposer"/> class to register the <see cref="ProtectConfiguredError404PagesComponent"/> class.
    /// </summary>
    [RuntimeLevel(MinLevel = RuntimeLevel.Run)]
    public class ProtectConfiguredError404PagesComponentComposer : ComponentComposer<ProtectConfiguredError404PagesComponent>
    {
    }
}