DEV: Add glimmer header deprecation notice (#25891)

- Remove `_customHeaderClasses`. It was not supposed to have been a part of the glimmer header PR
- Add deprecation notice to `addHeaderPanel`

# Example

<img width="437" alt="Screenshot 2024-02-26 at 2 50 39 PM" src="https://github.com/discourse/discourse/assets/50783505/23fce1ee-fff9-4071-8c34-a480cec7e2e7">

app/assets/javascripts/discourse/app/components/glimmer-header.gjs

@@ -4,7 +4,6 @@ import { action } from "@ember/object";
 import didInsert from "@ember/render-modifiers/modifiers/did-insert";
 import { inject as service } from "@ember/service";
 import { modifier } from "ember-modifier";
-import concatClass from "discourse/helpers/concat-class";
 import scrollLock from "discourse/lib/scroll-lock";
 import DiscourseURL from "discourse/lib/url";
 import { scrollTop } from "discourse/mixins/scroll-top";
@@ -21,11 +20,6 @@ import PluginOutlet from "./plugin-outlet";
 
 const SEARCH_BUTTON_ID = "search-button";
 
-let _customHeaderClasses = [];
-export function addCustomHeaderClass(className) {
-  _customHeaderClasses.push(className);
-}
-
 export default class GlimmerHeader extends Component {
   @service router;
   @service search;
@@ -53,10 +47,6 @@ export default class GlimmerHeader extends Component {
     };
   });
 
-  get customHeaderClasses() {
-    return _customHeaderClasses.join(" ");
-  }
-
   @action
   headerKeyboardTrigger(msg) {
     switch (msg.type) {
@@ -168,10 +158,7 @@ export default class GlimmerHeader extends Component {
   }
 
   <template>
-    <header
+    <header class="d-header" {{this.appEventsListeners}}>
-      class={{concatClass this.customHeaderClasses "d-header"}}
-      {{this.appEventsListeners}}
-    >
       <div class="wrap">
         <Contents
           @sidebarEnabled={{@sidebarEnabled}}

app/assets/javascripts/discourse/app/lib/plugin-api.js

@@ -9,7 +9,6 @@ import {
 import { addPluginDocumentTitleCounter } from "discourse/components/d-document";
 import { addToolbarCallback } from "discourse/components/d-editor";
 import { addCategorySortCriteria } from "discourse/components/edit-category-settings";
-import { addCustomHeaderClass } from "discourse/components/glimmer-header";
 import { addToHeaderIcons as addToGlimmerHeaderIcons } from "discourse/components/glimmer-header/icons";
 import { forceDropdownForMenuPanels as glimmerForceDropdownForMenuPanels } from "discourse/components/glimmer-site-header";
 import { addGlobalNotice } from "discourse/components/global-notice";
@@ -147,6 +146,18 @@ import { modifySelectKit } from "select-kit/mixins/plugin-api";
 
 export const PLUGIN_API_VERSION = "1.27.0";
 
+const DEPRECATED_HEADER_WIDGETS = [
+  "header",
+  "site-header",
+  "header-contents",
+  "header-buttons",
+  "user-status-bubble",
+  "sidebar-toggle",
+  "header-icons",
+  "header-topic-info",
+  "header-notifications",
+];
+
 // This helper prevents us from applying the same `modifyClass` over and over in test mode.
 function canModify(klass, type, resolverName, changes) {
   if (!changes.pluginId) {
@@ -185,6 +196,19 @@ function wrapWithErrorHandler(func, messageKey) {
   };
 }
 
+function deprecatedHeaderWidgetOverride(widgetName, override) {
+  if (DEPRECATED_HEADER_WIDGETS.includes(widgetName)) {
+    deprecated(
+      `The ${widgetName} widget has been deprecated and ${override} is no longer a supported override.`,
+      {
+        since: "v3.3.0.beta1-dev",
+        id: "discourse.header-widget-overrides",
+        url: "https://meta.discourse.org/t/296544",
+      }
+    );
+  }
+}
+
 class PluginApi {
   constructor(version, container) {
     this.version = version;
@@ -528,6 +552,9 @@ class PluginApi {
    *
    **/
   decorateWidget(name, fn) {
+    const widgetName = name.split(":")[0];
+    deprecatedHeaderWidgetOverride(widgetName, "decorateWidget");
+
     decorateWidget(name, fn);
   }
 
@@ -557,6 +584,8 @@ class PluginApi {
       return;
     }
 
+    deprecatedHeaderWidgetOverride(widget, "attachWidgetAction");
+
     widgetClass.prototype[actionName] = fn;
   }
 
@@ -874,6 +903,7 @@ class PluginApi {
    *
    **/
   changeWidgetSetting(widgetName, settingName, newValue) {
+    deprecatedHeaderWidgetOverride(widgetName, "changeWidgetSetting");
     changeSetting(widgetName, settingName, newValue);
   }
 
@@ -907,6 +937,7 @@ class PluginApi {
    **/
 
   reopenWidget(name, args) {
+    deprecatedHeaderWidgetOverride(name, "reopenWidget");
     return reopenWidget(name, args);
   }
 
@@ -934,10 +965,13 @@ class PluginApi {
    *
    **/
   addHeaderPanel(name, toggle, transformAttrs) {
-    // deprecated(
+    deprecated(
-    //   "addHeaderPanel has been removed. Use api.addToHeaderIcons instead.",
+      "addHeaderPanel has been removed. Use api.addToHeaderIcons instead.",
-    //   { id: "discourse.add-header-panel" }
+      {
-    // );
+        id: "discourse.add-header-panel",
+        url: "https://meta.discourse.org/t/296544",
+      }
+    );
     attachAdditionalPanel(name, toggle, transformAttrs);
   }
 
@@ -2802,19 +2836,6 @@ class PluginApi {
     addImageWrapperButton(label, btnClass, icon);
     addApiImageWrapperButtonClickEvent(fn);
   }
-
-  /**
-   * Add a custom css class to the header. The class or classes will live alongside the `d-header` class.
-   *
-   * ```
-   * api.addCustomHeaderClass("class-one");
-   * api.addCustomHeaderClass("class-two");
-   *
-   */
-
-  addCustomHeaderClass(klass) {
-    addCustomHeaderClass(klass);
-  }
 }
 
 // from http://stackoverflow.com/questions/6832596/how-to-compare-software-version-number-using-js-only-number