Ø So what are its origins? Where did it first appear? I haven't dug into this, but I imagine it was at Berkeley. What was it used for early on at individual sites?

It was certainly present in Version 6 UNIX, so it predates Berkeley.    While it being a “file” is very UNIX-ish, the concept of a settable sign on message doesn’t originate with UNIX.    I had used other systems that ran a user defined program on user login (sort of a compiled .profile) and it was common to put the system “news” in such.

One amusing thing to do with /etc/motd is to add the like “You might have mail.”    I thought it was a cute joke, but I never realized how much confusion it would cause.     I did it at BRL and had people sending me email asking why they didn’t have mail (it only said you MIGHT).

I told one of my student programmers working for me at Rutgers that if he put that in the motd on one of our systems I guaranteed within an hour someone will tell us they didn’t have mail.
It took about 15 minutes for one of my SENIOR systems programmers to come into the office and tell me that.