Spoilers are one of those areas where there’s just no hard-and-fast rules, and different folks react differently. My feeling (as poster and as moderator) is that well-known historical facts aren’t “spoilers” in an historical discussion. So, in a discussion about the historic Napoleon, no one will be upset to learn that he is defeated in the end.
However, if the discussion is about some dramatization of historical facts, then there are (to my mind) two different areas where spoiler tags might be necessary: (1) little-known historical facts (e.g., stuff that would NOT be taught in a high school history class) ; and (2) the dramatic way that the movie/play/tvshow reveals the historical facts.
Using the example of Spielberg’s 2012 movie LINCOLN:
(1) the specifics of the debate about slavery are very dramatic in the movie, and although mostly historical, the details of the behind-the-scenes manipulations are fascinating. It would NOT be a spoiler to reveal that Lincoln issued the Emancipation Proclamation (well-known historical fact) but it would be a spoiler to reveal the dramatic details of the political intrigues (historical facts, but NOT well-known).
(2) It’s not a “spoiler” that Lincoln is assassinated while watching a play at Ford’s Theatre, everyone knows that. But what would be a spoiler is the WAY that Spielberg dramatizes it and plays with our expectations by having us watching a different play at a different theatre