Yeah I'm sure we're all up to date on our 9th century viking history. Little different than Lincoln.
Can you spoil something that is regarded as to have happened in history? *Warning* Potential spoilers follow, I guess based on your view.
I understand you can spoil fiction, because there are no other avenues for learning of the conclusion of the story other than the piece of fiction itself. While Vikings is not completely historically accurate, it does base itself on English History and the Viking Sagas, and thus the conclusion of many of the characters can be gleamed from historical knowledge. Rollo becomes the founder of Norman Dynasty and Ragnar's death is what spurs the Viking invasion of England, resulting in unification of England under King Alfred, this is all regarded as fact. The quote from the show, "how the piglets would grunt if they knew how the old boar suffered," are believed to be Ragnar's final words.
The Last Kingdom on the other hand, while also taking place during the same time period, centers around a fictitious main character, whose conclusion we can not know from sources other than the material produced by the author. But the narrative, the unification of England under Alfred is the basis for the tale, and it is common knowledge that England is one country and not ruled by the Danes.
So I would argue that a person could spoil the Last Kingdom, because of the fictitious nature of the main character, but could not spoil the milestones of the series, because they are based on history, such as the death and titles of the characters who are actual people. If you instead argue that a person can spoil a series because the history is not common knowledge, who defines common knowledge? Is Spartacus dying at the end of Spartacus a spoiler? How about Roger Bannister running a 4 minute mile at the end of 4 minutes?
tl;dr You cant spoil history