config: add stub allowDeprecatedx86_64Darwin option

ci/eval/outpaths.nix

@@ -33,6 +33,9 @@ let
             allowVariants = !attrNamesOnly;
             checkMeta = true;
 
+            # Silence the `x86_64-darwin` deprecation warning.
+            allowDeprecatedx86_64Darwin = true;
+
             handleEvalIssue =
               reason: errormsg:
               let

pkgs/top-level/config.nix

@@ -431,6 +431,23 @@ let
         Please read https://www.visualstudio.com/license-terms/mt644918/ and enable this config if you accept.
       '';
     };
+
+    allowDeprecatedx86_64Darwin = mkOption {
+      # `force` does nothing; it’s reserved for forward compatibility
+      # with 26.11. We hide it from the documentation to avoid a
+      # footgun, as it will make the error in 26.11 less useful.
+      type = types.either types.bool (types.enum [ "force" ]) // {
+        inherit (types.bool) description descriptionClass;
+      };
+      default = false;
+      description = ''
+        Silence the warning for the upcoming deprecation of the
+        `x86_64-darwin` platform in Nixpkgs 26.11.
+
+        This does nothing in 25.11, and is provided there for forward
+        compatibility of configurations with 26.05.
+      '';
+    };
   };
 
 in