The Order definitely knows their history at least back to the Great Hyperspace War, considering how our Empire is just a continuation of that one and how people talk about Naga Sadow semi-regularly. According to Darth Baras the first few weeks of Sith training are supposed to be devoted to philosophy, I don't know that there's any history lessons involved, but I think it's still reasonable to assume that knowledge of Naga Sadow is very widespread.
The Hundred Year Darkness is definitely still known as well by the Sith, though probably more by important Darths than just every little acolyte. I imagine information about that has been preserved quite well simply because of how important it was, even Palpatine and beyond remembered what happened so it's significant enough to last thousands of years.
Before then it probably gets fragmented as you go further back, like ancient Assyria maybe. We know generic info like their legal systems, the extent of their territory, tax forms and applications, what inventions can be attributed to them, and how they dressed, but we do not know specific names for all of their kings or all the conflicts they fought in for example. Obviously that's not as glamorous, so events from pre-Sith Order times are probably not super commonly known unless the individual is really interested in history.
I'm sure the legend of King Adas lives on too, though he's probably semi-obscure and not entirely understood now kinda like Sargon of Akkad. Historians probably know that he united the Sith and drove some unknown alien invaders off of Korriban at least. I doubt people care about Dathka Graush that much, he seems more like a Naram-Sin than a Sargon II. One of his fortresses shows up in a crew skill mission tho, so there's that.
ICly I think a really historically interested Lord or Darth could have vague knowledge of anything between Adas's reign and today, though there's a lot of specifics that would definitely be left out. Like the Infinite Empire is really obscure and fairly controversial in this time period, basically fringe science, and there's not much linking it to the Sith in-universe anyway like relics or crashed ships, so you wouldn't be referring to the Rakata when telling the story of King Adas as an example.
So basically, it's probably not common for people to know ancient stuff, but that's just because most Sith aren't going to be interested unless it benefits them rather than because the information isn't available. Most Sith would probably be focusing on force techniques, politics, martial skill, etc instead. I think you could definitely justify knowing about any major event in Sith history as a Sith Lord or a Reclamation Service member tho, you'd just have to determine how much is reasonably discoverable in-universe.
And I agree with everything Bling said too. Especially purebloods being way more obsessive about history than everyone else.
