diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index ac389c2b5f..694e5f91b9 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Deprecate `ActiveSupport::Multibyte::Unicode.default_normalization_form`.
+
+    *Rafael Mendonça França*
+
 *   Remove deprecated `ActiveSupport::Multibyte::Unicode.pack_graphemes`,
     `ActiveSupport::Multibyte::Unicode.unpack_graphemes`,
     `ActiveSupport::Multibyte::Unicode.normalize`,
diff --git a/activesupport/lib/active_support/multibyte/unicode.rb b/activesupport/lib/active_support/multibyte/unicode.rb
index 072824339f..761cb6f741 100644
--- a/activesupport/lib/active_support/multibyte/unicode.rb
+++ b/activesupport/lib/active_support/multibyte/unicode.rb
@@ -5,20 +5,20 @@ module ActiveSupport
     module Unicode
       extend self
 
-      # A list of all available normalization forms.
-      # See https://www.unicode.org/reports/tr15/tr15-29.html for more
-      # information about normalization.
-      NORMALIZATION_FORMS = [:c, :kc, :d, :kd]
-
       # The Unicode version that is supported by the implementation
       UNICODE_VERSION = RbConfig::CONFIG["UNICODE_VERSION"]
 
-      # The default normalization used for operations that require
-      # normalization. It can be set to any of the normalizations
-      # in NORMALIZATION_FORMS.
-      #
-      #   ActiveSupport::Multibyte::Unicode.default_normalization_form = :c
-      attr_accessor :default_normalization_form # TODO: Deprecate
+      def default_normalization_form
+        ActiveSupport::Deprecation.warn(
+          "ActiveSupport::Multibyte::Unicode.default_normalization_form is deprecated and will be removed in Rails 6.1"
+        )
+      end
+
+      def default_normalization_form=(_)
+        ActiveSupport::Deprecation.warn(
+          "ActiveSupport::Multibyte::Unicode.default_normalization_form= is deprecated and will be removed in Rails 6.1"
+        )
+      end
 
       # Decompose composed characters to the decomposed form.
       def decompose(type, codepoints)
diff --git a/guides/source/6_1_release_notes.md b/guides/source/6_1_release_notes.md
index eb74f1bb2c..880380b315 100644
--- a/guides/source/6_1_release_notes.md
+++ b/guides/source/6_1_release_notes.md
@@ -314,6 +314,8 @@ Please refer to the [Changelog][active-support] for detailed changes.
 
 ### Deprecations
 
+*   Deprecate `ActiveSupport::Multibyte::Unicode.default_normalization_form`.
+
 ### Notable changes
 
 Active Job