diff --git a/lib/rubygems/commands/generate_index_command.rb b/lib/rubygems/commands/generate_index_command.rb
index 4cb3985..a11c761 100644
--- a/lib/rubygems/commands/generate_index_command.rb
+++ b/lib/rubygems/commands/generate_index_command.rb
@@ -22,14 +22,10 @@ def initialize
     end
 
     add_option "--[no-]modern",
-               "Generate indexes for RubyGems",
-               "(always true)" do |value, options|
+               "Generate indexes for RubyGems" do |value, options|
       options[:build_modern] = value
     end
 
-    deprecate_option("--modern", version: "4.0", extra_msg: "Modern indexes (specs, latest_specs, and prerelease_specs) are always generated, so this option is not needed.")
-    deprecate_option("--no-modern", version: "4.0", extra_msg: "The `--no-modern` option is currently ignored. Modern indexes (specs, latest_specs, and prerelease_specs) are always generated.")
-
     add_option "--[no-]compact",
                 "Generate compact index files" do |value, options|
       options[:build_compact] = value
@@ -73,8 +69,10 @@ def description # :nodoc:
   end
 
   def execute
-    # This is always true because it's the only way now.
-    options[:build_modern] = true
+    if !options[:build_modern] && !options[:build_compact]
+      alert_error "At least one of --modern or --compact must be enabled."
+      terminate_interaction 1
+    end
 
     if !File.exist?(options[:directory]) ||
        !File.directory?(options[:directory])
diff --git a/lib/rubygems/indexer.rb b/lib/rubygems/indexer.rb
index 65fa768..cbcc434 100644
--- a/lib/rubygems/indexer.rb
+++ b/lib/rubygems/indexer.rb
@@ -108,8 +108,12 @@ def initialize(directory, options = {})
   def build_indices
     specs = map_gems_to_specs gem_file_list
     Gem::Specification._resort! specs
-    build_marshal_gemspecs specs
-    build_modern_indices specs if @build_modern
+
+    if @build_modern
+      build_marshal_gemspecs specs
+      build_modern_indices specs
+    end
+
     build_compact_index specs if @build_compact
 
     compress_indices
@@ -288,10 +292,10 @@ def map_gems_to_specs(gems)
   # All future files should be compressed using gzip, not deflate
 
   def compress_indices
-    say "Compressing indices"
+    if @build_modern
+      say "Compressing indices"
 
-    Gem.time "Compressed indices" do
-      if @build_modern
+      Gem.time "Compressed indices" do
         gzip @specs_index
         gzip @latest_specs_index
         gzip @prerelease_specs_index
@@ -400,7 +404,7 @@ def install_indices
   def make_temp_directories
     FileUtils.rm_rf @directory
     FileUtils.mkdir_p @directory, :mode => 0o700
-    FileUtils.mkdir_p @quick_marshal_dir
+    FileUtils.mkdir_p @quick_marshal_dir if @build_modern
   end
 
   ##
@@ -421,7 +425,13 @@ def paranoid(path, extension)
   def update_index
     make_temp_directories
 
-    specs_mtime = File.stat(@dest_specs_index).mtime
+    if @build_modern
+      reference_file = @dest_specs_index
+    elsif @build_compact
+      reference_file = File.join(@dest_directory, "versions")
+    end
+
+    specs_mtime = File.stat(reference_file).mtime
     newest_mtime = Time.at 0
 
     updated_gems = gem_file_list.select do |gem|
@@ -438,14 +448,18 @@ def update_index
     specs = map_gems_to_specs updated_gems
     prerelease, released = specs.partition {|s| s.version.prerelease? }
 
-    files = build_marshal_gemspecs specs
+    files = []
+
+    if @build_modern
+      files = build_marshal_gemspecs specs
 
-    Gem.time "Updated indexes" do
-      update_specs_index released, @dest_specs_index, @specs_index
-      update_specs_index released, @dest_latest_specs_index, @latest_specs_index
-      update_specs_index(prerelease,
-                         @dest_prerelease_specs_index,
-                         @prerelease_specs_index)
+      Gem.time "Updated indexes" do
+        update_specs_index released, @dest_specs_index, @specs_index
+        update_specs_index released, @dest_latest_specs_index, @latest_specs_index
+        update_specs_index(prerelease,
+                           @dest_prerelease_specs_index,
+                           @prerelease_specs_index)
+      end
     end
 
     if @build_compact
@@ -460,12 +474,14 @@ def update_index
 
     say "Updating production dir #{@dest_directory}" if verbose
 
-    files << @specs_index
-    files << "#{@specs_index}.gz"
-    files << @latest_specs_index
-    files << "#{@latest_specs_index}.gz"
-    files << @prerelease_specs_index
-    files << "#{@prerelease_specs_index}.gz"
+    if @build_modern
+      files << @specs_index
+      files << "#{@specs_index}.gz"
+      files << @latest_specs_index
+      files << "#{@latest_specs_index}.gz"
+      files << @prerelease_specs_index
+      files << "#{@prerelease_specs_index}.gz"
+    end
 
     files = files.map do |path|
       path.sub(%r{^#{Regexp.escape @directory}/?}, "") # HACK?
diff --git a/test/rubygems/test_gem_commands_generate_index_command.rb b/test/rubygems/test_gem_commands_generate_index_command.rb
index 7e719a5..993bad8 100644
--- a/test/rubygems/test_gem_commands_generate_index_command.rb
+++ b/test/rubygems/test_gem_commands_generate_index_command.rb
@@ -60,22 +60,45 @@ def test_handle_options_update
   end
 
   def test_handle_options_modern
-    use_ui @ui do
-      @cmd.handle_options %w[--modern]
-    end
+    @cmd.handle_options %w[--modern]
 
-    assert_equal \
-      "WARNING:  The \"--modern\" option has been deprecated and will be removed in Rubygems 4.0. Modern indexes (specs, latest_specs, and prerelease_specs) are always generated, so this option is not needed.\n",
-      @ui.error
+    assert @cmd.options[:build_modern]
   end
 
   def test_handle_options_no_modern
+    @cmd.handle_options %w[--no-modern]
+
+    refute @cmd.options[:build_modern]
+  end
+
+  def test_execute_compact_only
+    @cmd.options[:build_modern] = false
+    @cmd.options[:build_compact] = true
+
     use_ui @ui do
-      @cmd.handle_options %w[--no-modern]
+      @cmd.execute
+    end
+
+    names = File.join @gemhome, "names"
+    assert File.exist?(names), names
+
+    versions = File.join @gemhome, "versions"
+    assert File.exist?(versions), versions
+
+    specs = File.join @gemhome, "specs.4.8.gz"
+    refute File.exist?(specs), specs
+  end
+
+  def test_execute_no_modern_no_compact
+    @cmd.options[:build_modern] = false
+    @cmd.options[:build_compact] = false
+
+    assert_raise Gem::MockGemUi::TermError do
+      use_ui @ui do
+        @cmd.execute
+      end
     end
 
-    assert_equal \
-      "WARNING:  The \"--no-modern\" option has been deprecated and will be removed in Rubygems 4.0. The `--no-modern` option is currently ignored. Modern indexes (specs, latest_specs, and prerelease_specs) are always generated.\n",
-      @ui.error
+    assert_match(/At least one of --modern or --compact must be enabled/, @ui.error)
   end
 end
diff --git a/test/rubygems/test_gem_indexer.rb b/test/rubygems/test_gem_indexer.rb
index c86c83e..5975fa4 100644
--- a/test/rubygems/test_gem_indexer.rb
+++ b/test/rubygems/test_gem_indexer.rb
@@ -504,4 +504,127 @@ def file_md5(file)
   def file_sha256(file)
     Digest::SHA256.base64digest(Gem.read_binary(file))
   end
+
+  def test_build_indices_compact_only
+    with_indexer(@indexerdir, build_compact: true, build_modern: false) do |indexer|
+      indexer.make_temp_directories
+
+      use_ui @ui do
+        indexer.build_indices
+      end
+
+      assert File.exist?(File.join(indexer.directory, "names")), "names file should exist"
+      assert File.exist?(File.join(indexer.directory, "versions")), "versions file should exist"
+      assert_directory_exists File.join(indexer.directory, "info")
+
+      refute File.exist?(File.join(indexer.directory, "specs.#{@marshal_version}")),
+             "specs index should not exist"
+      refute File.exist?(File.join(indexer.directory, "latest_specs.#{@marshal_version}")),
+             "latest_specs index should not exist"
+      refute File.exist?(File.join(indexer.directory, "prerelease_specs.#{@marshal_version}")),
+             "prerelease_specs index should not exist"
+      refute File.directory?(File.join(indexer.directory, "quick")),
+             "quick dir should not exist"
+    end
+  end
+
+  def test_generate_index_compact_only
+    with_indexer(@indexerdir, build_compact: true, build_modern: false) do |indexer|
+      use_ui @ui do
+        indexer.generate_index
+      end
+
+      assert_indexed @indexerdir, "names"
+      assert_indexed @indexerdir, "versions"
+      assert_directory_exists File.join(@indexerdir, "info")
+
+      refute_indexed @indexerdir, "specs.#{@marshal_version}"
+      refute_indexed @indexerdir, "specs.#{@marshal_version}.gz"
+      refute_indexed @indexerdir, "latest_specs.#{@marshal_version}"
+      refute_indexed @indexerdir, "latest_specs.#{@marshal_version}.gz"
+      refute_indexed @indexerdir, "prerelease_specs.#{@marshal_version}"
+      refute_indexed @indexerdir, "prerelease_specs.#{@marshal_version}.gz"
+
+      quickdir = File.join @indexerdir, "quick"
+      refute File.directory?(quickdir), "quick directory should not exist"
+
+      refute_directory_exists indexer.directory
+    end
+  end
+
+  def test_generate_index_compact_only_ui
+    with_indexer(@indexerdir, build_compact: true, build_modern: false) do |indexer|
+      use_ui @ui do
+        indexer.generate_index
+      end
+
+      refute_match(/Generating Marshal quick index/, @ui.output)
+      refute_match(/Generating specs index/, @ui.output)
+      refute_match(/Generating latest specs index/, @ui.output)
+      refute_match(/Generating prerelease specs index/, @ui.output)
+      refute_match(/Compressing indices/, @ui.output)
+
+      assert_match(/Generating compact index/, @ui.output)
+    end
+  end
+
+  def test_update_index_compact_only
+    with_indexer(@indexerdir, build_compact: true, build_modern: false) do |indexer|
+      use_ui @ui do
+        indexer.generate_index
+      end
+    end
+
+    assert_indexed @indexerdir, "names"
+    assert_indexed @indexerdir, "versions"
+    infodir = File.join @indexerdir, "info"
+    assert_directory_exists infodir
+
+    versions_file = File.read File.join @indexerdir, "versions"
+    info_d_file = File.read File.join @indexerdir, "info", "d"
+
+    @d2_1 = util_spec "d", "2.1"
+    util_build_gem @d2_1
+
+    @e1 = util_spec "e", "1"
+    util_build_gem @e1
+
+    gems = File.join @indexerdir, "gems"
+    FileUtils.mv @d2_1.cache_file, gems
+    FileUtils.mv @e1.cache_file, gems
+
+    with_indexer(@indexerdir, build_compact: true, build_modern: false) do |indexer|
+      with_system_gems do
+        use_ui @ui do
+          indexer.update_index
+        end
+
+        assert_indexed infodir, "e"
+        assert_indexed infodir, "d"
+
+        assert_equal <<~INFO_FILE, File.read(File.join(infodir, "e"))
+          ---
+          1 |checksum:#{file_sha256(File.join(gems, "e-1.gem"))}
+        INFO_FILE
+
+        assert_equal <<~INFO_FILE, File.read(File.join(infodir, "d"))
+          #{info_d_file.chomp}
+          2.1 |checksum:#{file_sha256(File.join(gems, "d-2.1.gem"))}
+        INFO_FILE
+
+        assert_equal <<~VERSIONS_FILE, File.read(File.join(@indexerdir, "versions"))
+          #{versions_file.chomp}
+          d 2.1 #{file_md5(File.join(@indexerdir, "info", "d"))}
+          e 1 #{file_md5(File.join(@indexerdir, "info", "e"))}
+        VERSIONS_FILE
+
+        assert_versions_file_info_checksums(@indexerdir)
+
+        refute_indexed @indexerdir, "specs.#{@marshal_version}"
+        refute_indexed @indexerdir, "specs.#{@marshal_version}.gz"
+        refute File.directory?(File.join(@indexerdir, "quick")),
+               "quick directory should not exist"
+      end
+    end
+  end
 end