Add deprecation notice to 3p iframe f.js

3p/integration.js

@@ -28,6 +28,7 @@ import './polyfills';
 import {draw3p, init} from './integration-lib';
 import {getMode} from '../src/mode';
 import {register} from './3p';
+import {user} from 'src/log';
 
 // 3P - please keep in alphabetic order
 import {beopinion} from './beopinion';
@@ -311,6 +312,11 @@ import {zen} from '#ads/vendors/zen';
 import {zergnet} from '#ads/vendors/zergnet';
 import {zucks} from '#ads/vendors/zucks';
 
+user().warn(
+  '3P-IFRAME',
+  'f.js in iframe is being deprecated (see https://github.com/ampproject/amphtml/issues/35349 for details)'
+);
+
 init(window);
 
 if (getMode().test || getMode().localDev) {

3p/remote.html

@@ -6,7 +6,7 @@
 (function() {
 var v = location.search.substr(1);
 if (!(/^\d+(-canary)?$/.test(v))) return;
-var u = 'https://3p.ampproject.net/'+encodeURIComponent(v)+'/f.js';
+var u = 'https://3p.ampproject.net/'+encodeURIComponent(v)+'/_ping_.js'; // Replace _ping_ with the vendor you wish to use
 document.write('<script'+' src="'+encodeURI(u)+'"><'+'/script>'); // lgtm [js/eval-like-call]
 })();
 </script>