Socket activated services first appeared in inetd(8) somewhere in the eighties and is as UNIX as it gets :)
I'd hardly call it services, in terms of what inetd could do to keep the _service_, more like wrappers, but yeah. Systemd made it better - I run both xinetd based bash scripts for sorta network-accessible healthchecks used by haproxy later and 8000+ systemd's socket activated services, can compare apples to apples.