Earlier quoted context omitted.
The history of the .plan file goes back to way before ID or Carmack. It's been part of unix for a long time. Back when things were simpler, you could finger a user and read their .plan to see what they were up to.
... and other useful info such as office location, phone number, etc. Those were the days before corporate directories.
Hmm, I wonder if I ask my Mastodon host nicely if they'd open up finger protocol support, lol. (ETA: Reminds one of the old adage about blind men touching the elephant.)