show error in legacy short url create endpoint

src/plugins/share/server/url_service/http/register_url_service_routes.ts

@@ -13,6 +13,7 @@ import { registerGetRoute } from './short_urls/register_get_route';
 import { registerDeleteRoute } from './short_urls/register_delete_route';
 import { registerResolveRoute } from './short_urls/register_resolve_route';
 import { registerGotoRoute } from './short_urls/register_goto_route';
+import { registerShortenUrlRoute } from './short_urls/register_shorten_url_route';
 
 export const registerUrlServiceRoutes = (
   core: CoreSetup,
@@ -24,4 +25,5 @@ export const registerUrlServiceRoutes = (
   registerDeleteRoute(router, url);
   registerResolveRoute(router, url);
   registerGotoRoute(router, core);
+  registerShortenUrlRoute(router, core);
 };

src/plugins/share/server/url_service/http/short_urls/register_shorten_url_route.ts

@@ -0,0 +1,23 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+import { CoreSetup, IRouter } from 'kibana/server';
+
+export const registerShortenUrlRoute = (router: IRouter, core: CoreSetup) => {
+  core.http.resources.register(
+    {
+      path: '/api/shorten_url',
+      validate: {},
+    },
+    router.handleLegacyErrors(async (ctx, req, res) => {
+      return res.badRequest({
+        body: 'This endpoint is no longer supported. Please use the new URL shortening service.',
+      });
+    })
+  );
+};