diff --git a/packages/@ember/-internals/deprecations/index.ts b/packages/@ember/-internals/deprecations/index.ts
index 560d1355405..855ecf206a3 100644
--- a/packages/@ember/-internals/deprecations/index.ts
+++ b/packages/@ember/-internals/deprecations/index.ts
@@ -129,6 +129,16 @@ export const DEPRECATIONS = {
       enabled: '5.10.0',
     },
   }),
+  DEPRECATE_ARRAY_PROTOTYPE_EXTENSIONS: deprecation({
+    id: 'deprecate-array-prototype-extensions',
+    url: 'https://deprecations.emberjs.com/id/deprecate-deprecate-array-prototype-extensions',
+    until: '6.0.0',
+    for: 'ember-source',
+    since: {
+      available: '5.10.0',
+      enabled: '5.10.0',
+    },
+  }),
 };
 
 export function deprecateUntil(message: string, deprecation: DeprecationObject) {
diff --git a/packages/@ember/-internals/environment/lib/env.ts b/packages/@ember/-internals/environment/lib/env.ts
index ae161ca5a39..6547cbe9e6d 100644
--- a/packages/@ember/-internals/environment/lib/env.ts
+++ b/packages/@ember/-internals/environment/lib/env.ts
@@ -1,3 +1,4 @@
+import { DEPRECATIONS, deprecateUntil } from '@ember/-internals/deprecations';
 import { DEBUG } from '@glimmer/env';
 import global from './global';
 
@@ -202,6 +203,14 @@ export const ENV = {
   }
 
   let { EXTEND_PROTOTYPES } = EmberENV;
+
+  if (EXTEND_PROTOTYPES !== false) {
+    deprecateUntil(
+      'Array prototype extensions are deprecated. Follow the deprecation guide for migration instructions, and set EmberENV.EXTEND_PROTOTYPES to false in your config/environment.js',
+      DEPRECATIONS.DEPRECATE_ARRAY_PROTOTYPE_EXTENSIONS
+    );
+  }
+
   if (EXTEND_PROTOTYPES !== undefined) {
     if (typeof EXTEND_PROTOTYPES === 'object' && EXTEND_PROTOTYPES !== null) {
       ENV.EXTEND_PROTOTYPES.Array = EXTEND_PROTOTYPES.Array !== false;