nixos/systemd: drop support for legacy cgroup hierachy

nixos/modules/system/boot/systemd.nix

@@ -341,14 +341,6 @@ in
       '';
     };
 
-    enableUnifiedCgroupHierarchy = mkOption {
-      default = true;
-      type = types.bool;
-      description = ''
-        Whether to enable the unified cgroup hierarchy (cgroupsv2); see {manpage}`cgroups(7)`.
-      '';
-    };
-
     extraConfig = mkOption {
       default = "";
       type = types.lines;
@@ -694,12 +686,6 @@ in
     # https://github.com/systemd/systemd/pull/12226
     boot.kernel.sysctl."kernel.pid_max" = mkIf pkgs.stdenv.is64bit (lib.mkDefault 4194304);
 
-    boot.kernelParams = optional (!cfg.enableUnifiedCgroupHierarchy) "systemd.unified_cgroup_hierarchy=0";
-
-    # Avoid potentially degraded system state due to
-    # "Userspace Out-Of-Memory (OOM) Killer was skipped because of a failed condition check (ConditionControlGroupController=v2)."
-    systemd.oomd.enable = mkIf (!cfg.enableUnifiedCgroupHierarchy) false;
-
     services.logrotate.settings = {
       "/var/log/btmp" = mapAttrs (_: mkDefault) {
         frequency = "monthly";
@@ -723,5 +709,10 @@ in
       (mkRenamedOptionModule [ "boot" "systemd" "services" ] [ "systemd" "services" ])
       (mkRenamedOptionModule [ "jobs" ] [ "systemd" "services" ])
       (mkRemovedOptionModule [ "systemd" "generator-packages" ] "Use systemd.packages instead.")
+      (mkRemovedOptionModule ["systemd" "enableUnifiedCgroupHierarchy"] ''
+          In 256 support for cgroup v1 ('legacy' and 'hybrid' hierarchies) is now considered obsolete and systemd by default will refuse to boot under it.
+          To forcibly reenable cgroup v1 support, you can set boot.kernelParams = [ "systemd.unified_cgroup_hierachy=0" "SYSTEMD_CGROUP_ENABLE_LEGACY_FORCE=1" ].
+          NixOS does not officially support this configuration and might cause your system to be unbootable in future versions. You are on your own.
+      '')
     ];
 }