diff --git a/Gemfile.lock b/Gemfile.lock
index b1f9991adc5da..8e219f3f31463 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -71,7 +71,7 @@ PATH
       i18n (>= 0.7, < 2)
       minitest (~> 5.1)
       tzinfo (~> 1.1)
-      zeitwerk (~> 2.0)
+      zeitwerk (~> 2.1)
     rails (6.0.0.beta3)
       actioncable (= 6.0.0.beta3)
       actionmailbox (= 6.0.0.beta3)
@@ -526,7 +526,7 @@ GEM
     websocket-extensions (0.1.3)
     xpath (3.2.0)
       nokogiri (~> 1.8)
-    zeitwerk (2.0.0)
+    zeitwerk (2.1.0)
 
 PLATFORMS
   java
diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index cf72673dbb2ed..57e03b5e12f3b 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,11 @@
+*   The Zeitwerk compatibility interface for `ActiveSupport::Dependencies` no
+    longer implements `autoloaded_constants` or `autoloaded?` (undocumented,
+    anyway). Experience shows introspection does not have many use cases, and
+    troubleshooting is done by logging. With this design trade-off we are able
+    to use even less memory in all environments.
+
+    *Xavier Noria*
+
 *   Depends on Zeitwerk 2, which stores less metadata if reloading is disabled
     and hence uses less memory when `config.cache_classes` is `true`, a standard
     setup in production.
diff --git a/activesupport/activesupport.gemspec b/activesupport/activesupport.gemspec
index e719fc6176042..546cc1797af06 100644
--- a/activesupport/activesupport.gemspec
+++ b/activesupport/activesupport.gemspec
@@ -34,5 +34,5 @@ Gem::Specification.new do |s|
   s.add_dependency "tzinfo",          "~> 1.1"
   s.add_dependency "minitest",        "~> 5.1"
   s.add_dependency "concurrent-ruby", "~> 1.0", ">= 1.0.2"
-  s.add_dependency "zeitwerk",        "~> 2.0"
+  s.add_dependency "zeitwerk",        "~> 2.1"
 end
diff --git a/activesupport/lib/active_support/dependencies/zeitwerk_integration.rb b/activesupport/lib/active_support/dependencies/zeitwerk_integration.rb
index faf9edef27c57..b0f7a3f9cc187 100644
--- a/activesupport/lib/active_support/dependencies/zeitwerk_integration.rb
+++ b/activesupport/lib/active_support/dependencies/zeitwerk_integration.rb
@@ -21,17 +21,8 @@ def safe_constantize(cpath)
           ActiveSupport::Inflector.safe_constantize(cpath)
         end
 
-        def autoloaded_constants
-          cpaths = []
-          Rails.autoloaders.each do |autoloader|
-            cpaths.concat(autoloader.loaded_cpaths.to_a)
-          end
-          cpaths
-        end
-
-        def autoloaded?(object)
-          cpath = object.is_a?(Module) ? object.name : object.to_s
-          Rails.autoloaders.any? { |autoloader| autoloader.loaded?(cpath) }
+        def to_unload?(cpath)
+          Rails.autoloaders.main.to_unload?(cpath)
         end
 
         def verbose=(verbose)
diff --git a/activesupport/lib/active_support/descendants_tracker.rb b/activesupport/lib/active_support/descendants_tracker.rb
index 2dca99071222c..fe0c6991aa867 100644
--- a/activesupport/lib/active_support/descendants_tracker.rb
+++ b/activesupport/lib/active_support/descendants_tracker.rb
@@ -22,11 +22,18 @@ def descendants(klass)
 
       def clear
         if defined? ActiveSupport::Dependencies
+          # to_unload? is only defined in Zeitwerk mode.
+          to_unload = if Dependencies.respond_to?(:to_unload?)
+            ->(klass) { Dependencies.to_unload?(klass.name) }
+          else
+            ->(klass) { Dependencies.autoloaded?(klass) }
+          end
+
           @@direct_descendants.each do |klass, descendants|
-            if ActiveSupport::Dependencies.autoloaded?(klass)
+            if to_unload[klass]
               @@direct_descendants.delete(klass)
             else
-              descendants.reject! { |v| ActiveSupport::Dependencies.autoloaded?(v) }
+              descendants.reject! { |v| to_unload[v] }
             end
           end
         else
diff --git a/railties/test/application/zeitwerk_integration_test.rb b/railties/test/application/zeitwerk_integration_test.rb
index a9da06034735d..b461d82450503 100644
--- a/railties/test/application/zeitwerk_integration_test.rb
+++ b/railties/test/application/zeitwerk_integration_test.rb
@@ -98,24 +98,35 @@ class RESTfulController < ApplicationController
     assert_nil deps.safe_constantize("Admin")
   end
 
-  test "autoloaded_constants returns autoloaded constant paths" do
-    app_file "app/models/admin/user.rb", "class Admin::User; end"
+  test "to_unload? says if a constant would be unloaded (main)" do
+    app_file "app/models/user.rb", "class User; end"
     app_file "app/models/post.rb", "class Post; end"
     boot
 
-    assert Admin::User
-    assert_equal ["Admin", "Admin::User"], deps.autoloaded_constants
+    assert Post
+    assert deps.to_unload?("Post")
+    assert_not deps.to_unload?("User")
+  end
+
+  test "to_unload? says if a constant would be unloaded (once)" do
+    add_to_config 'config.autoload_once_paths << "#{Rails.root}/extras"'
+    app_file "extras/foo.rb", "class Foo; end"
+    app_file "extras/bar.rb", "class Bar; end"
+    boot
+
+    assert Foo
+    assert_not deps.to_unload?("Foo")
+    assert_not deps.to_unload?("Bar")
   end
 
-  test "autoloaded? says if a constant has been autoloaded" do
+  test "to_unload? says if a constant would be unloaded (reloading disabled)" do
     app_file "app/models/user.rb", "class User; end"
     app_file "app/models/post.rb", "class Post; end"
-    boot
+    boot("production")
 
     assert Post
-    assert deps.autoloaded?("Post")
-    assert deps.autoloaded?(Post)
-    assert_not deps.autoloaded?("User")
+    assert_not deps.to_unload?("Post")
+    assert_not deps.to_unload?("User")
   end
 
   test "eager loading loads the application code" do
@@ -315,4 +326,45 @@ def once_autoloader.reload
       assert_nil autoloader.logger
     end
   end
+
+  # This is here because to guarantee classic mode works as always, Zeitwerk
+  # integration does not touch anything in classic. The descendants tracker is a
+  # very small one-liner exception. We leave its main test suite untouched, and
+  # add some minimal safety net here.
+  #
+  # When time passes, things are going to be reorganized (famous last words).
+  test "descendants tracker" do
+    class ::ZeitwerkDTIntegrationTestRoot
+      extend ActiveSupport::DescendantsTracker
+    end
+    class ::ZeitwerkDTIntegrationTestChild < ::ZeitwerkDTIntegrationTestRoot; end
+    class ::ZeitwerkDTIntegrationTestGrandchild < ::ZeitwerkDTIntegrationTestChild; end
+
+    begin
+      app_file "app/models/user.rb", "class User < ZeitwerkDTIntegrationTestRoot; end"
+      app_file "app/models/post.rb", "class Post < ZeitwerkDTIntegrationTestRoot; end"
+      app_file "app/models/tutorial.rb", "class Tutorial < Post; end"
+      boot
+
+      assert User
+      assert Tutorial
+
+      direct_descendants = [ZeitwerkDTIntegrationTestChild, User, Post].to_set
+      assert_equal direct_descendants, ZeitwerkDTIntegrationTestRoot.direct_descendants.to_set
+
+      descendants = direct_descendants.merge([ZeitwerkDTIntegrationTestGrandchild, Tutorial])
+      assert_equal descendants, ZeitwerkDTIntegrationTestRoot.descendants.to_set
+
+      ActiveSupport::DescendantsTracker.clear
+
+      direct_descendants = [ZeitwerkDTIntegrationTestChild].to_set
+      assert_equal direct_descendants, ZeitwerkDTIntegrationTestRoot.direct_descendants.to_set
+
+      descendants = direct_descendants.merge([ZeitwerkDTIntegrationTestGrandchild])
+      assert_equal descendants, ZeitwerkDTIntegrationTestRoot.descendants.to_set
+    ensure
+      Object.send(:remove_const, :ZeitwerkDTIntegrationTestRoot)
+      Object.send(:remove_const, :ZeitwerkDTIntegrationTestChild)
+    end
+  end
 end