For the curious: The furthest back that I have managed to trace it so far is 1986, using Usenet archives. A patch that was posted to mod.sources on 1986-08-01 (v06i083, Michael A. Callahan) apparently references getopt() argument parsing and mentions "ps -U", implying that 4.2 BSD ps was using getopt() by then. (Although there's no direct mention of getopt and this may be wrong.)

* https://groups.google.com/d/msg/mod.sources/H-shkbdVIbs/VJUI...