@@ -1344,12 +1344,15 @@ removed. Please use `sloppy` instead.
 ### DEP0066: outgoingMessage.\_headers, outgoingMessage.\_headerNames
 <!-- YAML
 changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/24167
+    description: Runtime deprecation.
   - version: v8.0.0
     pr-url: https://github.com/nodejs/node/pull/10941
     description: Documentation-only deprecation.
 -->
 
-Type: Documentation-only
+Type: Runtime
 
 The `http` module `outgoingMessage._headers` and `outgoingMessage._headerNames`
 properties are deprecated. Use one of the public methods
 
@@ -110,10 +110,10 @@ util.inherits(OutgoingMessage, Stream);
 
 
 Object.defineProperty(OutgoingMessage.prototype, '_headers', {
-  get: function() {
+  get: util.deprecate(function() {
     return this.getHeaders();
-  },
-  set: function(val) {
+  }, 'OutgoingMessage.prototype._headers is deprecated', 'DEP0066'),
+  set: util.deprecate(function(val) {
     if (val == null) {
       this[outHeadersKey] = null;
     } else if (typeof val === 'object') {
@@ -124,11 +124,11 @@ Object.defineProperty(OutgoingMessage.prototype, '_headers', {
         headers[name.toLowerCase()] = [name, val[name]];
       }
     }
-  }
+  }, 'OutgoingMessage.prototype._headers is deprecated', 'DEP0066')
 });
 
 Object.defineProperty(OutgoingMessage.prototype, '_headerNames', {
-  get: function() {
+  get: util.deprecate(function() {
     const headers = this[outHeadersKey];
     if (headers !== null) {
       const out = Object.create(null);
@@ -141,8 +141,8 @@ Object.defineProperty(OutgoingMessage.prototype, '_headerNames', {
       return out;
     }
     return null;
-  },
-  set: function(val) {
+  }, 'OutgoingMessage.prototype._headerNames is deprecated', 'DEP0066'),
+  set: util.deprecate(function(val) {
     if (typeof val === 'object' && val !== null) {
       const headers = this[outHeadersKey];
       if (!headers)
@@ -154,7 +154,7 @@ Object.defineProperty(OutgoingMessage.prototype, '_headerNames', {
           header[0] = val[keys[i]];
       }
     }
-  }
+  }, 'OutgoingMessage.prototype._headerNames is deprecated', 'DEP0066')
 });
 
 
 
@@ -0,0 +1,13 @@
+'use strict';
+const common = require('../common');
+
+const { OutgoingMessage } = require('http');
+
+const warn = 'OutgoingMessage.prototype._headerNames is deprecated';
+common.expectWarning('DeprecationWarning', warn, 'DEP0066');
+
+{
+  // tests for _headerNames get method
+  const outgoingMessage = new OutgoingMessage();
+  outgoingMessage._headerNames;
+}
@@ -0,0 +1,15 @@
+'use strict';
+const common = require('../common');
+
+const { OutgoingMessage } = require('http');
+
+const warn = 'OutgoingMessage.prototype._headerNames is deprecated';
+common.expectWarning('DeprecationWarning', warn, 'DEP0066');
+
+{
+  // tests for _headerNames set method
+  const outgoingMessage = new OutgoingMessage();
+  outgoingMessage._headerNames = {
+    'x-flow-id': '61bba6c5-28a3-4eab-9241-2ecaa6b6a1fd'
+  };
+}
@@ -6,6 +6,9 @@ const assert = require('assert');
 const { outHeadersKey } = require('internal/http');
 const { OutgoingMessage } = require('http');
 
+const warn = 'OutgoingMessage.prototype._headers is deprecated';
+common.expectWarning('DeprecationWarning', warn, 'DEP0066');
+
 {
   // tests for _headers get method
   const outgoingMessage = new OutgoingMessage();
Original file line number	Diff line number	Diff line change
`@@ -6,6 +6,9 @@ const assert = require('assert');`
`6`	`6`	`const { outHeadersKey } = require('internal/http');`
`7`	`7`	`const { OutgoingMessage } = require('http');`
`8`	`8`
	`9`	`+const warn = 'OutgoingMessage.prototype._headers is deprecated';`
	`10`	`+common.expectWarning('DeprecationWarning', warn, 'DEP0066');`
	`11`	`+`
`9`	`12`	`{`
`10`	`13`	`// tests for _headers get method`
`11`	`14`	`const outgoingMessage = new OutgoingMessage();`