They're not planning to kill her off. They're just going to retire her, possibly demote her to non-speaking roles. Who knows, maybe they'll re-cast her later down the line like with Lunchlady Doris.
They
are planning to kill a character off
obviously, it's going to be a secret until the episode airs. Whoever it winds up being, it's probably going to wind up a minor player for the sake of an episode plotline. I can't imagine them killing Sideshow Bob off permanently. The thing that's actually chilling is that they
mentioned Edna in the article, and this was reported a while ago.
Of the three main cast members who died, the problem lies in what they're going to do with Edna. While Lionel Hutz and Troy MacClure are quite missed and Doris was eventually recast, they weren't major players in the series. Edna is Bart's fourth grade teacher, and a
lot of the show's humor and tension stems from Bart's school room antics. Also unfortunate is her role was actually expanding in the show up to this point. While it is respectful to retire the character, ultimately the better option is to recast her so the legacy of the character lives on and to continue the show's major characters.