This, I did not know. I thought the killing of the Maude was just a bit of Simpsons non sequitur, but no. Check it out, learned while reading about the current contract dispute:
Producers did kill off Maude Flanders' character -- she was hit by a T-shirt shot from a gun and fell from a stadium grandstand -- when actress Maggie Roswell demanded more money in 1999. Yet it's highly unlikely that 20th Century Fox would seek to replace the lead roles, because new voices would seem jarring to fans.