The Last Kingdom has done a fair amount of tinkering with the source material's timeline in order to make its adaptation work, leading to the premature deaths of these 5 characters. The Last Kingdom, based on Bernard Cornwell's novel series, The Saxon Stories, ended its 5-season run in 2022 before wrapping up the story with a Netflix original film, The Last Kingdom: Seven Kings Must Die. The series has become a sensation for Netflix since it was acquired in full from the BBC in 2018.
https://screenrant.com/last-kingdom-characters-still-alive-books/