Good points, Henning! (Welcome by the way)
A couple things to consider though...
There are such things as historians. Even if the masses weren't familiar with ancient Sith symbols, the historians would be. And I'm sure they would've brought it to attention. And honestly, it wouyldn't even take historians. You said to dscribe a redcoats uniform without looking it up, but the thing is people in the GFFA can look it up.
With your French flag you used, I'll admit I had no idea that was what their flag looked like. However the fleur-de-lis, a commonly known French symbol, is prominently featured on it. I expect the ancient Sith logo (is that what you would call it?) would be much the same. The exact flag? No. The symbols? Yes.
Palpatine removing the opposition makes it slightly more credible, but I'm still not buying it. Imagine trying to eliminate the memory of a well-known culture. Think about all the sources (books, newspapers, magazines, movies, documentaries, encyclopedias, databases etc) that have information about this culture. And in Star Wars, we're not just dealing with one world, but an entire galaxy.
Where sky and water meet,
Where the waves grow sweet,
Doubt not, Reepicheep,
To find all you seek,
There is the utter east.