diff --git a/CHANGELOG b/CHANGELOG
index 14ba7576b..17caf366e 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,7 +1,9 @@
-# 3.8.1 (2023-XX-XX)
+# 3.9.0 (2023-XX-XX)
 
  * Add return type for Symfony 7 compatibility
  * Fix premature loop exit in Security Policy lookup of allowed methods/properties
+ * Deprecate all internal extension functions in favor of methods on the extension classes
+ * Mark all extension functions as @internal
 
 # 3.8.0 (2023-11-21)
 
diff --git a/composer.json b/composer.json
index 1b1726fe8..31a248c2b 100644
--- a/composer.json
+++ b/composer.json
@@ -26,6 +26,7 @@
     "require": {
         "php": ">=7.2.5",
         "symfony/polyfill-php80": "^1.22",
+        "symfony/deprecation-contracts": "^2.5|^3",
         "symfony/polyfill-mbstring": "^1.3",
         "symfony/polyfill-ctype": "^1.8"
     },
diff --git a/doc/deprecated.rst b/doc/deprecated.rst
index a5771ef4e..948dfb3ef 100644
--- a/doc/deprecated.rst
+++ b/doc/deprecated.rst
@@ -10,3 +10,10 @@ Functions
 
  * The `twig_test_iterable` function is deprecated; use the native
    `is_iterable` instead.
+
+Extensions
+----------
+
+* All functions defined in Twig extensions are marked as internal as of Twig
+  3.9.0, and will be removed in Twig 4.0. They have been replaced by internal
+  methods on their respective extension classes.