diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6d7ac5d5..0242ec05 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,8 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 
 ### Deprecated
 
+- Deprecate `OutputFormat::sSpaceAfterListArgumentSeparator` being an array
+  (#876)
 - Deprecate `OutputFormat::level()` (#870)
 
 ### Removed
diff --git a/src/OutputFormat.php b/src/OutputFormat.php
index 2ce257fb..df5d6a87 100644
--- a/src/OutputFormat.php
+++ b/src/OutputFormat.php
@@ -103,7 +103,9 @@ class OutputFormat
     public $sSpaceBeforeListArgumentSeparator = '';
 
     /**
-     * @var string
+     * #866 `array` is deprecated in version V8.8.0, will be changed to `string` only in V9.0.0.
+     *
+     * @var string|array<string, string>
      */
     public $sSpaceAfterListArgumentSeparator = '';
 
diff --git a/tests/OutputFormatTest.php b/tests/OutputFormatTest.php
index ff543a9c..060a3be8 100644
--- a/tests/OutputFormatTest.php
+++ b/tests/OutputFormatTest.php
@@ -104,6 +104,8 @@ public function spaceAfterListArgumentSeparator()
 
     /**
      * @test
+     *
+     * @deprecated #866 since 8.8.0, will be removed in 9.0.0.
      */
     public function spaceAfterListArgumentSeparatorComplex()
     {