Sith was never said in the OT…
Actually, there is a deleted scene from the conference room where a Sith Lord is mentioned:
https://www.youtube.com/watch?v=8Erf6s_wYJkYeah, I’ve seen that clip. Good stuff, should’ve been in the film.
It’s unlikely the internet nerds of 1977 were able to pass the clip around, though…so “Sith” was still definitely unknown to the majority of the world until Phantom.
“Sith” was on trading cards and toys that were released even prior to A New Hope. It didn’t spread over the internet, but it was known by the nerds and collectors of the time. Same with Palpatine’s name.