'Simpsons' Exec -- Edna's NOT the Mystery Death

E.P. Al Jean tells us ... they never planned that Edna would be the character to bite it. In fact, he says they had been writing a storyline based on her newly-wedded life with Ned Flanders.

We're told Marcia had been unable to do "Simpsons" voice over work for the past few weeks due to her declining health ... but producers were still hoping for the best.

Al tells us the Edna character will more than likely be retired this season -- but it's too early to say how she'll go away.

Al adds, "Marcia was such a sweet and talented woman ... she will be missed."