haskellPackages.nixfmt: drop

Nixfmt is no longer released to Hackage; the package there is the
unmaintained "Nixfmt Classic" which is being dropped in Nixpkgs 26.11.

pkgs/development/haskell-modules/configuration-common.nix

@@ -720,8 +720,12 @@ with haskellLib;
   }) super.shell-conduit;
 
   # No maintenance planned until eventual removal
+  # Throw added 2026-08-19
   # https://github.com/NixOS/nixfmt/issues/340#issuecomment-3315920564
-  nixfmt = doJailbreak super.nixfmt;
+  nixfmt =
+    lib.throwIf pkgs.config.allowAliases
+      "haskell.packages.*.nixfmt has been removed as it is deprecated and unmaintained. Consider using top-level nixfmt instead."
+      (doJailbreak super.nixfmt);
 
   # Too strict upper bounds on turtle and text
   # https://github.com/awakesecurity/nix-deploy/issues/35

pkgs/development/haskell-modules/configuration-hackage2nix/main.yaml

@@ -856,6 +856,12 @@ dont-distribute-packages:
   # Depends on shine, which is a ghcjs project.
   - shine-varying
 
+  # these packages are deprecated or unmaintained upstream
+  # TODO: fully exclude. See:
+  # - https://github.com/NixOS/cabal2nix/pull/667
+  # - https://github.com/NixOS/nixpkgs/pull/441204
+  - nixfmt # 2026-08-18 maintained versions are not released on hackage
+
   # these packages depend on software with an unfree license
   - accelerate-bignum
   - accelerate-blas

pkgs/development/haskell-modules/hackage-packages.nix

@@ -496548,6 +496548,7 @@ self: {
       ];
       description = "An opinionated formatter for Nix";
       license = lib.licenses.mpl20;
+      hydraPlatforms = lib.platforms.none;
       mainProgram = "nixfmt";
     }
   ) { };

pkgs/top-level/aliases.nix

@@ -1784,7 +1784,7 @@ mapAliases {
   nix-plugin-pijul = throw "nix-plugin-pijul has been removed due to being discontinued"; # Added 2025-05-18
   nix_2_3 = throw "'nix_2_3' has been removed, because it was unmaintained and insecure."; # Converted to throw 2025-07-24
   nixbang = throw "'nixbang' has been removed because it was unmaintained upstream. Use nix-shell shebang instead'"; # Added 2026-07-02
-  nixfmt-classic = throw "nixfmt-classic has been removed as it is deprecated and unmaintained." haskellPackages.nixfmt.bin; # Converted to throw 2026-07-01
+  nixfmt-classic = throw "nixfmt-classic has been removed as it is deprecated and unmaintained."; # Converted to throw 2026-07-01
   nixfmt-rfc-style = warnAlias "nixfmt-rfc-style is now the same as pkgs.nixfmt which should be used instead." nixfmt; # Added 2025-07-14
   nixForLinking = throw "nixForLinking has been removed, use `nixVersions.nixComponents_<version>` instead"; # Added 2025-08-14
   nixnote2 = throw "'nixnote2' has been removed as upstream has been unmaintained since 2017"; # Added 2026-04-26