diff --git a/lib/net/http.rb b/lib/net/http.rb
index a5834412..7e89409c 100644
--- a/lib/net/http.rb
+++ b/lib/net/http.rb
@@ -1221,16 +1221,9 @@ def proxy_port
       end
     end
 
-    # [Bug #12921]
-    if /linux|freebsd|darwin/ =~ RUBY_PLATFORM
-      ENVIRONMENT_VARIABLE_IS_MULTIUSER_SAFE = true
-    else
-      ENVIRONMENT_VARIABLE_IS_MULTIUSER_SAFE = false
-    end
-
     # The username of the proxy server, if one is configured.
     def proxy_user
-      if ENVIRONMENT_VARIABLE_IS_MULTIUSER_SAFE && @proxy_from_env
+      if @proxy_from_env
         user = proxy_uri&.user
         unescape(user) if user
       else
@@ -1240,7 +1233,7 @@ def proxy_user
 
     # The password of the proxy server, if one is configured.
     def proxy_pass
-      if ENVIRONMENT_VARIABLE_IS_MULTIUSER_SAFE && @proxy_from_env
+      if @proxy_from_env
         pass = proxy_uri&.password
         unescape(pass) if pass
       else
diff --git a/test/net/http/test_http.rb b/test/net/http/test_http.rb
index e9471273..0508645a 100644
--- a/test/net/http/test_http.rb
+++ b/test/net/http/test_http.rb
@@ -178,13 +178,8 @@ def test_proxy_eh_ENV_with_user
       http = Net::HTTP.new 'hostname.example'
 
       assert_equal true, http.proxy?
-      if Net::HTTP::ENVIRONMENT_VARIABLE_IS_MULTIUSER_SAFE
-        assert_equal 'foo', http.proxy_user
-        assert_equal 'bar', http.proxy_pass
-      else
-        assert_nil http.proxy_user
-        assert_nil http.proxy_pass
-      end
+      assert_equal 'foo', http.proxy_user
+      assert_equal 'bar', http.proxy_pass
     end
   end
 
@@ -195,13 +190,8 @@ def test_proxy_eh_ENV_with_urlencoded_user
       http = Net::HTTP.new 'hostname.example'
 
       assert_equal true, http.proxy?
-      if Net::HTTP::ENVIRONMENT_VARIABLE_IS_MULTIUSER_SAFE
-        assert_equal "Y\\X", http.proxy_user
-        assert_equal "R%S] ?X", http.proxy_pass
-      else
-        assert_nil http.proxy_user
-        assert_nil http.proxy_pass
-      end
+      assert_equal "Y\\X", http.proxy_user
+      assert_equal "R%S] ?X", http.proxy_pass
     end
   end