diff --git a/app/assets/javascripts/discourse/app/instance-initializers/boot-client-error-handler.js b/app/assets/javascripts/discourse/app/instance-initializers/boot-services.js
similarity index 62%
rename from app/assets/javascripts/discourse/app/instance-initializers/boot-client-error-handler.js
rename to app/assets/javascripts/discourse/app/instance-initializers/boot-services.js
index d9f4480b9f4..44c7b39deb6 100644
--- a/app/assets/javascripts/discourse/app/instance-initializers/boot-client-error-handler.js
+++ b/app/assets/javascripts/discourse/app/instance-initializers/boot-services.js
@@ -1,5 +1,6 @@
 export default {
   initialize(owner) {
     owner.lookup("service:client-error-handler");
+    owner.lookup("service:deprecation-warning-handler");
   },
 };
diff --git a/app/assets/javascripts/discourse/app/services/deprecation-warning-handler.js b/app/assets/javascripts/discourse/app/services/deprecation-warning-handler.js
new file mode 100644
index 00000000000..3483e60cc4c
--- /dev/null
+++ b/app/assets/javascripts/discourse/app/services/deprecation-warning-handler.js
@@ -0,0 +1,111 @@
+import { registerDeprecationHandler } from "@ember/debug";
+import Service, { inject as service } from "@ember/service";
+import { addGlobalNotice } from "discourse/components/global-notice";
+import identifySource from "discourse/lib/source-identifier";
+import { escapeExpression } from "discourse/lib/utilities";
+import { registerDeprecationHandler as registerDiscourseDeprecationHandler } from "discourse-common/lib/deprecated";
+import { bind } from "discourse-common/utils/decorators";
+import I18n from "discourse-i18n";
+
+// Deprecations matching patterns on this list will trigger warnings for admins.
+// To avoid 'crying wolf', we should only add values here when we're sure they're
+// not being triggered by core or official themes/plugins.
+export const CRITICAL_DEPRECATIONS = [
+  /^discourse.modal-controllers$/,
+  /^(?!discourse\.)/, // All unsilenced ember deprecations
+];
+
+// Deprecation handling APIs don't have any way to unregister handlers, so we set up permenant
+// handlers and link them up to the application lifecycle using module-local state.
+let handler;
+registerDeprecationHandler((message, opts, next) => {
+  handler?.(message, opts);
+  return next(message, opts);
+});
+registerDiscourseDeprecationHandler((message, opts) =>
+  handler?.(message, opts)
+);
+
+export default class DeprecationWarningHandler extends Service {
+  @service currentUser;
+  @service siteSettings;
+
+  #adminWarned = false;
+
+  constructor() {
+    super(...arguments);
+    handler = this.handle;
+  }
+
+  willDestroy() {
+    handler = null;
+  }
+
+  @bind
+  handle(message, opts) {
+    const workflowConfigs = window.deprecationWorkflow?.config?.workflow;
+    const matchingConfig = workflowConfigs.find(
+      (config) => config.matchId === opts.id
+    );
+
+    if (matchingConfig && matchingConfig.handler === "silence") {
+      return;
+    }
+
+    const source = identifySource();
+    if (source?.type === "browser-extension") {
+      return;
+    }
+
+    this.maybeNotifyAdmin(opts.id, source);
+  }
+
+  maybeNotifyAdmin(id, source) {
+    if (this.#adminWarned) {
+      return;
+    }
+
+    if (!this.currentUser?.admin) {
+      return;
+    }
+
+    if (!this.siteSettings.warn_critical_js_deprecations) {
+      return;
+    }
+
+    if (CRITICAL_DEPRECATIONS.some((pattern) => pattern.test(id))) {
+      this.notifyAdmin(id, source);
+    }
+  }
+
+  notifyAdmin(id, source) {
+    this.#adminWarned = true;
+
+    let notice = I18n.t("critical_deprecation.notice");
+
+    if (this.siteSettings.warn_critical_js_deprecations_message) {
+      notice += " " + this.siteSettings.warn_critical_js_deprecations_message;
+    }
+
+    if (source?.type === "theme") {
+      notice +=
+        " " +
+        I18n.t("critical_deprecation.theme_source", {
+          name: escapeExpression(source.name),
+          path: source.path,
+        });
+    } else if (source?.type === "plugin") {
+      notice +=
+        " " +
+        I18n.t("critical_deprecation.plugin_source", {
+          name: escapeExpression(source.name),
+        });
+    }
+
+    addGlobalNotice(notice, "critical-deprecation", {
+      dismissable: true,
+      dismissDuration: moment.duration(1, "day"),
+      level: "warn",
+    });
+  }
+}
diff --git a/config/locales/client.en.yml b/config/locales/client.en.yml
index a2c57341b3e..544879944de 100644
--- a/config/locales/client.en.yml
+++ b/config/locales/client.en.yml
@@ -225,6 +225,11 @@ en:
 
     broken_plugin_alert: "Caused by plugin '%{name}'"
 
+    critical_deprecation:
+      notice: "<b>[Admin Notice]</b> One of your themes or plugins needs updating for compatibility with upcoming Discourse core changes (<a target='_blank' href='https://meta.discourse.org/t/287211'>more info</a>)."
+      theme_source: "Identified theme: <a target='_blank' href='%{path}'>'%{name}'</a>."
+      plugin_source: "Identified plugin: '%{name}'"
+
     s3:
       regions:
         ap_northeast_1: "Asia Pacific (Tokyo)"
diff --git a/config/site_settings.yml b/config/site_settings.yml
index 675f99ade1e..1f22b96a238 100644
--- a/config/site_settings.yml
+++ b/config/site_settings.yml
@@ -2308,6 +2308,14 @@ developer:
     default: false
     client: true
     hidden: true
+  warn_critical_js_deprecations:
+    default: true
+    client: true
+    hidden: true
+  warn_critical_js_deprecations_message:
+    default: ""
+    client: true
+    hidden: true
 
 navigation:
   navigation_menu:
diff --git a/spec/system/ember_deprecation_spec.rb b/spec/system/ember_deprecation_spec.rb
index 37779bece3f..738d4c642b7 100644
--- a/spec/system/ember_deprecation_spec.rb
+++ b/spec/system/ember_deprecation_spec.rb
@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
-describe "Production mode debug shims", type: :system do
-  it "can successfully print a deprecation message after applying prod shims" do
+describe "JS Deprecation Handling", type: :system do
+  it "can successfully print a deprecation message after applying production-mode shims" do
     visit("/latest")
     expect(find("#main-outlet-wrapper")).to be_visible
 
@@ -31,4 +31,25 @@ describe "Production mode debug shims", type: :system do
     expect(call).to eq("DEPRECATION: Some message [deprecation id: some.id]")
     expect(backtrace).to include("shimLogDeprecationToConsole")
   end
+
+  it "shows warnings to admins for critical deprecations" do
+    sign_in Fabricate(:admin)
+
+    SiteSetting.warn_critical_js_deprecations = true
+    SiteSetting.warn_critical_js_deprecations_message =
+      "Discourse core changes will be applied to your site on Jan 15."
+
+    visit("/latest")
+
+    page.execute_script <<~JS
+      const deprecated = require("discourse-common/lib/deprecated").default;
+      deprecated("Fake deprecation message", { id: "fake-deprecation" })
+    JS
+
+    message = find("#global-notice-critical-deprecation")
+    expect(message).to have_text(
+      "One of your themes or plugins needs updating for compatibility with upcoming Discourse core changes",
+    )
+    expect(message).to have_text(SiteSetting.warn_critical_js_deprecations_message)
+  end
 end