I am trying to remember if the Rebel Alliance and later New Republic did heavy PR stuff with the Heroes of Yavin and that they had a Jedi take out the first Death Star and latter defeat the Emperor on the Second Death Star? I know they had to fight through the heavy Imperial propaganda to even get out that either Death Star was destroyed (or even existed) and that the Emperor was dead (I don't know how public the likes of Darth Vader was to the general public....Palatine being a Sith Lord was basically a state secret, right?)
Pretty much. Those in his inner circle knew, though to those that weren't Sith cultists it didn't really matter. The Sith would have been even less well understood than the Jedi and after a millennia those that know the name at all either associate it with myths, fairy tales or obscure historical curiosities. It'd be like someone today claiming to be a Celtic Pagan. Most would just shrug, because who cares?
I think the existence of "Darth Sidious" was a secret less out of a desire to keep people from knowing he was a Sith specifically, and more to just surround Palpatine's actual appearance and activities in secrecy. He wasn't exactly an "Emperor of the People" and hardly ever appeared in public, with most public imagery of him being heavily doctored.
Similarly the existence of Vader wasn't exactly a state secret either, but the paranoid and secretive nature of the inner goings on of any totalitarian regime means they very little information about him got out beyond the upper echelons. Any military personnel that dealt with him personally (and lived) probably knew better than to gossip between the ranks.
Ironically, the Rebel Alliance probably knew more about both of them than the average citizen, though I suspect a lot of that was kept secret by them too. There's always the problem that if they let on everything they know, then it'll get back to ISB and then come questions like "how did they know that?" and "who could possibly have that information?" and suddenly Bail and by extension Leia's parentage are at risk of exposure.
As for rebel propaganda; they did publish an art book a few years back featuring just this and while there are some images featuring Vader, most of it is squarely focused on x-wing pilots and a few about Leia. IIRC nothing about Luke specifically or even the Jedi in general. It makes sense since the Jedi were vilified by both sides by the end of the Clone Wars and making the son on General Skywalker the poster boy for the Alliance would probably alienate the factions that were either former separatists or anti-war republic loyalists.
I also like to think that Mon and perhaps even Leia nixed the idea out of concern for painting an even bigger target on Luke's back. There's probably still a very large bounty on the head on any suspected Jedi, and thanks to Kanan and Ezra the Rebellion have had some experience with Jedi in the ranks. The last thing they need is the Inquisitorius making things even more complicated for all concerned.