Didn't you have to show ID at check-in (which you could not do online) at least by the 90s? Maybe my memory's tainted by a high ratio of international flying.

That technically didn't prevent someone from giving away their boarding pass once they had it, of course.

No, not in US domestic flights in the 80s or 90s.

International was different since the airline had to check you had a passport (and visa if applicable) for the destination country.