Plot Armor tends to wear off in the final arc, episode, battle, etc. (Unmarked spoilers)
Plot Armor is what keeps important characters alive for plot purposes. This is when the Plot Armor comes off. Whenever the Grand Finale is here or coming, suddenly, Anyone Can Die. This is usually done to make the ending more suspenseful and exciting but it's also done to make the war seem more costly. There may be some overlap with Darker and Edgier. No Real Life Examples, Please!. Because this does not happen in Real Life. (and while we're at it, Plot Armor does not exist in Real Life either.) This is a Death Trope so beware unmarked spoilers!