diff --git a/packages/@ember/-internals/environment/lib/env.ts b/packages/@ember/-internals/environment/lib/env.ts
index 7e42a6f04ef..fab88cf6687 100644
--- a/packages/@ember/-internals/environment/lib/env.ts
+++ b/packages/@ember/-internals/environment/lib/env.ts
@@ -1,3 +1,4 @@
+import { FUNCTION_PROTOTYPE_EXTENSIONS } from '@ember/deprecated-features';
 import global from './global';
 
 /**
@@ -155,12 +156,16 @@ export const ENV = {
   if (EXTEND_PROTOTYPES !== undefined) {
     if (typeof EXTEND_PROTOTYPES === 'object' && EXTEND_PROTOTYPES !== null) {
       ENV.EXTEND_PROTOTYPES.String = EXTEND_PROTOTYPES.String !== false;
-      ENV.EXTEND_PROTOTYPES.Function = EXTEND_PROTOTYPES.Function !== false;
+      if (FUNCTION_PROTOTYPE_EXTENSIONS) {
+        ENV.EXTEND_PROTOTYPES.Function = EXTEND_PROTOTYPES.Function !== false;
+      }
       ENV.EXTEND_PROTOTYPES.Array = EXTEND_PROTOTYPES.Array !== false;
     } else {
       let isEnabled = EXTEND_PROTOTYPES !== false;
       ENV.EXTEND_PROTOTYPES.String = isEnabled;
-      ENV.EXTEND_PROTOTYPES.Function = isEnabled;
+      if (FUNCTION_PROTOTYPE_EXTENSIONS) {
+        ENV.EXTEND_PROTOTYPES.Function = isEnabled;
+      }
       ENV.EXTEND_PROTOTYPES.Array = isEnabled;
     }
   }
diff --git a/packages/@ember/-internals/metal/tests/observer_test.js b/packages/@ember/-internals/metal/tests/observer_test.js
index ffa4ed59cf3..d84c06438c4 100644
--- a/packages/@ember/-internals/metal/tests/observer_test.js
+++ b/packages/@ember/-internals/metal/tests/observer_test.js
@@ -16,6 +16,7 @@ import {
   set,
 } from '..';
 import { moduleFor, AbstractTestCase } from 'internal-test-helpers';
+import { FUNCTION_PROTOTYPE_EXTENSIONS } from '@ember/deprecated-features';
 
 function K() {}
 
@@ -103,18 +104,20 @@ moduleFor(
       assert.equal(observerCount, 10, 'should continue to fire indefinitely');
     }
 
-    ['@test observer added declaratively via brace expansion should fire when property changes'](
+    ['@test observer added via Function.prototype extensions and brace expansion should fire when property changes'](
       assert
     ) {
-      if (ENV.EXTEND_PROTOTYPES.Function) {
+      if (!FUNCTION_PROTOTYPE_EXTENSIONS && ENV.EXTEND_PROTOTYPES.Function) {
         let obj = {};
         let count = 0;
 
-        mixin(obj, {
-          observeFooAndBar: function() {
-            count++;
-          }.observes('{foo,bar}'),
-        });
+        expectDeprecation(() => {
+          mixin(obj, {
+            observeFooAndBar: function() {
+              count++;
+            }.observes('{foo,bar}'),
+          });
+        }, /Function prototype extensions have been deprecated, please migrate from function\(\){}.observes\('foo'\) to observer\('foo', function\(\) {}\)/);
 
         set(obj, 'foo', 'foo');
         assert.equal(count, 1, 'observer specified via brace expansion invoked on property change');
@@ -129,10 +132,10 @@ moduleFor(
       }
     }
 
-    ['@test observer specified declaratively via brace expansion should fire when dependent property changes'](
+    ['@test observer specified via Function.prototype extensions via brace expansion should fire when dependent property changes'](
       assert
     ) {
-      if (ENV.EXTEND_PROTOTYPES.Function) {
+      if (!FUNCTION_PROTOTYPE_EXTENSIONS && ENV.EXTEND_PROTOTYPES.Function) {
         let obj = { baz: 'Initial' };
         let count = 0;
 
@@ -152,11 +155,13 @@ moduleFor(
           })
         );
 
-        mixin(obj, {
-          fooAndBarWatcher: function() {
-            count++;
-          }.observes('{foo,bar}'),
-        });
+        expectDeprecation(() => {
+          mixin(obj, {
+            fooAndBarWatcher: function() {
+              count++;
+            }.observes('{foo,bar}'),
+          });
+        }, /Function prototype extensions have been deprecated, please migrate from function\(\){}.observes\('foo'\) to observer\('foo', function\(\) {}\)/);
 
         get(obj, 'foo');
         set(obj, 'baz', 'Baz');
diff --git a/packages/@ember/-internals/runtime/lib/ext/function.js b/packages/@ember/-internals/runtime/lib/ext/function.js
index b3af2ee7649..e6ae6a11f7f 100644
--- a/packages/@ember/-internals/runtime/lib/ext/function.js
+++ b/packages/@ember/-internals/runtime/lib/ext/function.js
@@ -4,8 +4,10 @@
 
 import { ENV } from '@ember/-internals/environment';
 import { on, computed, observer } from '@ember/-internals/metal';
+import { deprecate } from '@ember/debug';
+import { FUNCTION_PROTOTYPE_EXTENSIONS } from '@ember/deprecated-features';
 
-if (ENV.EXTEND_PROTOTYPES.Function) {
+if (FUNCTION_PROTOTYPE_EXTENSIONS && ENV.EXTEND_PROTOTYPES.Function) {
   Object.defineProperties(Function.prototype, {
     /**
       The `property` extension of Javascript's Function prototype is available
@@ -77,6 +79,15 @@ if (ENV.EXTEND_PROTOTYPES.Function) {
       enumerable: false,
       writable: true,
       value: function() {
+        deprecate(
+          `Function prototype extensions have been deprecated, please migrate from function(){}.property('bar') to computed('bar', function() {}).`,
+          false,
+          {
+            id: 'function-prototype-extensions.property',
+            until: '4.0.0',
+            url: 'https://deprecations.emberjs.com/v3.x#toc_function-prototype-extensions-property',
+          }
+        );
         return computed(...arguments, this);
       },
     },
@@ -113,6 +124,16 @@ if (ENV.EXTEND_PROTOTYPES.Function) {
       enumerable: false,
       writable: true,
       value: function() {
+        deprecate(
+          `Function prototype extensions have been deprecated, please migrate from function(){}.observes('foo') to observer('foo', function() {}).`,
+          false,
+          {
+            id: 'function-prototype-extensions.observes',
+            until: '4.0.0',
+            url: 'https://deprecations.emberjs.com/v3.x#toc_function-prototype-extensions-observes',
+          }
+        );
+
         return observer(...arguments, this);
       },
     },
@@ -147,6 +168,16 @@ if (ENV.EXTEND_PROTOTYPES.Function) {
       enumerable: false,
       writable: true,
       value: function() {
+        deprecate(
+          `Function prototype extensions have been deprecated, please migrate from function(){}.on('foo') to on('foo', function() {}).`,
+          false,
+          {
+            id: 'function-prototype-extensions.on',
+            until: '4.0.0',
+            url: 'https://deprecations.emberjs.com/v3.x#toc_function-prototype-extensions-on',
+          }
+        );
+
         return on(...arguments, this);
       },
     },
diff --git a/packages/@ember/-internals/runtime/tests/ext/function_test.js b/packages/@ember/-internals/runtime/tests/ext/function_test.js
index 68997ffa0ae..8ace67fe307 100644
--- a/packages/@ember/-internals/runtime/tests/ext/function_test.js
+++ b/packages/@ember/-internals/runtime/tests/ext/function_test.js
@@ -3,12 +3,13 @@ import { Mixin, mixin, get, set } from '@ember/-internals/metal';
 import EmberObject from '../../lib/system/object';
 import Evented from '../../lib/mixins/evented';
 import { moduleFor, AbstractTestCase } from 'internal-test-helpers';
+import { FUNCTION_PROTOTYPE_EXTENSIONS } from '@ember/deprecated-features';
 
 moduleFor(
   'Function.prototype.observes() helper',
   class extends AbstractTestCase {
     ['@test global observer helper takes multiple params'](assert) {
-      if (!ENV.EXTEND_PROTOTYPES.Function) {
+      if (!FUNCTION_PROTOTYPE_EXTENSIONS || !ENV.EXTEND_PROTOTYPES.Function) {
         assert.ok(
           'undefined' === typeof Function.prototype.observes,
           'Function.prototype helper disabled'
@@ -16,13 +17,16 @@ moduleFor(
         return;
       }
 
-      let MyMixin = Mixin.create({
-        count: 0,
+      let MyMixin;
+      expectDeprecation(() => {
+        MyMixin = Mixin.create({
+          count: 0,
 
-        foo: function() {
-          set(this, 'count', get(this, 'count') + 1);
-        }.observes('bar', 'baz'),
-      });
+          foo: function() {
+            set(this, 'count', get(this, 'count') + 1);
+          }.observes('bar', 'baz'),
+        });
+      }, /Function prototype extensions have been deprecated, please migrate from function\(\){}.observes\('foo'\) to observer\('foo', function\(\) {}\)/);
 
       let obj = mixin({}, MyMixin);
       assert.equal(get(obj, 'count'), 0, 'should not invoke observer immediately');
@@ -38,7 +42,7 @@ moduleFor(
   'Function.prototype.on() helper',
   class extends AbstractTestCase {
     ['@test sets up an event listener, and can trigger the function on multiple events'](assert) {
-      if (!ENV.EXTEND_PROTOTYPES.Function) {
+      if (!FUNCTION_PROTOTYPE_EXTENSIONS || !ENV.EXTEND_PROTOTYPES.Function) {
         assert.ok(
           'undefined' === typeof Function.prototype.on,
           'Function.prototype helper disabled'
@@ -46,13 +50,16 @@ moduleFor(
         return;
       }
 
-      let MyMixin = Mixin.create({
-        count: 0,
+      let MyMixin;
+      expectDeprecation(() => {
+        MyMixin = Mixin.create({
+          count: 0,
 
-        foo: function() {
-          set(this, 'count', get(this, 'count') + 1);
-        }.on('bar', 'baz'),
-      });
+          foo: function() {
+            set(this, 'count', get(this, 'count') + 1);
+          }.on('bar', 'baz'),
+        });
+      }, /Function prototype extensions have been deprecated, please migrate from function\(\){}.on\('foo'\) to on\('foo', function\(\) {}\)/);
 
       let obj = mixin({}, Evented, MyMixin);
       assert.equal(get(obj, 'count'), 0, 'should not invoke listener immediately');
@@ -63,19 +70,22 @@ moduleFor(
     }
 
     ['@test can be chained with observes'](assert) {
-      if (!ENV.EXTEND_PROTOTYPES.Function) {
+      if (!FUNCTION_PROTOTYPE_EXTENSIONS || !ENV.EXTEND_PROTOTYPES.Function) {
         assert.ok('Function.prototype helper disabled');
         return;
       }
 
-      let MyMixin = Mixin.create({
-        count: 0,
-        bay: 'bay',
-        foo: function() {
-          set(this, 'count', get(this, 'count') + 1);
-        }
-          .observes('bay')
-          .on('bar'),
+      let MyMixin;
+      expectDeprecation(function() {
+        MyMixin = Mixin.create({
+          count: 0,
+          bay: 'bay',
+          foo: function() {
+            set(this, 'count', get(this, 'count') + 1);
+          }
+            .observes('bay')
+            .on('bar'),
+        });
       });
 
       let obj = mixin({}, Evented, MyMixin);
@@ -92,7 +102,7 @@ moduleFor(
   'Function.prototype.property() helper',
   class extends AbstractTestCase {
     ['@test sets up a ComputedProperty'](assert) {
-      if (!ENV.EXTEND_PROTOTYPES.Function) {
+      if (!FUNCTION_PROTOTYPE_EXTENSIONS || !ENV.EXTEND_PROTOTYPES.Function) {
         assert.ok(
           'undefined' === typeof Function.prototype.property,
           'Function.prototype helper disabled'
@@ -100,13 +110,16 @@ moduleFor(
         return;
       }
 
-      let MyClass = EmberObject.extend({
-        firstName: null,
-        lastName: null,
-        fullName: function() {
-          return get(this, 'firstName') + ' ' + get(this, 'lastName');
-        }.property('firstName', 'lastName'),
-      });
+      let MyClass;
+      expectDeprecation(function() {
+        MyClass = EmberObject.extend({
+          firstName: null,
+          lastName: null,
+          fullName: function() {
+            return get(this, 'firstName') + ' ' + get(this, 'lastName');
+          }.property('firstName', 'lastName'),
+        });
+      }, /Function prototype extensions have been deprecated, please migrate from function\(\){}.property\('bar'\) to computed\('bar', function\(\) {}\)/);
 
       let obj = MyClass.create({ firstName: 'Fred', lastName: 'Flinstone' });
       assert.equal(get(obj, 'fullName'), 'Fred Flinstone', 'should return the computed value');
diff --git a/packages/@ember/deprecated-features/index.ts b/packages/@ember/deprecated-features/index.ts
index b4acfa4fdc7..417ad0ebcfb 100644
--- a/packages/@ember/deprecated-features/index.ts
+++ b/packages/@ember/deprecated-features/index.ts
@@ -13,3 +13,4 @@ export const COMPONENT_MANAGER_STRING_LOOKUP = !!'3.8.0';
 export const JQUERY_INTEGRATION = !!'3.9.0';
 export const ALIAS_METHOD = !!'3.9.0';
 export const APP_CTRL_ROUTER_PROPS = !!'3.10.0-beta.1';
+export const FUNCTION_PROTOTYPE_EXTENSIONS = !!'3.11.0-beta.1';