My point is that there really isn’t that much and you would be better off looking elsewhere for savings.
The only piece of software shipped by the systemd project that I personally consider unfit for purpose is systemd-timesyncd. Swapping that out for an NTP client maintained by people who take the challenges of time synchronization seriously is as simple as
services.timesyncd.enable = false;
services.chrony.enable = true;
# work around https://github.com/NixOS/nixpkgs/issues/445035
systemd.tmpfiles.rules = lib.mkAfter [
"z ${config.services.chrony.directory}/chrony.keys 0640 root chrony - -"
];
I do not know whether this is a net win or lose on memory consumption.
Because of https://www.openwall.com/lists/oss-security/2025/12/28/4 I also strongly recommend
systemd.generators.systemd-ssh-generator = "/dev/null";
systemd.sockets.sshd-unix-local.enable = lib.mkForce false;
systemd.sockets.sshd-vsock.enable = lib.mkForce false;
on any system that has sshd enabled, but again, this is not because of memory costs.