Wallace passed away suddenly in 2013, with Mrs Krabappel's final scenes airing just a few months later in the sitcom's 25th season.
Now, with The Simpsons into its 32nd season, the series has paid touching tribute to both Marcia and Edna, with the school teacher making "her last appearance" in its latest episode, 'Diary Queen'.
The episode, in which Bart finds an old diary of Mrs Krabappel, features two previously-used lines, signed off by and credited to Wallace's estate.
Opening up about the writers' decision to bring the character back, The Simpsons' executive producer Al Jean told Variety that the show "never got the chance to give a... proper goodbye".
"[Marcia] was so beloved by everybody that worked on The Simpsons, that we just sort of wanted to give her one last moment on the show in her memory," he explained. "It's just that, it's nothing more, but we thought she was the best."
Confirming that Mrs Krabappel will never return to Springfield, he added: "This will be her last appearance. We just wanted to sort of send the character off properly, in a way that I think isn't trying to be exploitive."
The producer also explained the mystery of Edna's death, admitting that he didn't think it was necessary for fans to know how a character had passed away.
As for Edna's involvement in Sunday's (February 21) episode, Jean added: "We revealed a little bit more about her, and then a little bit more about her life and how the character was feeling when she passed."
The Simpsons is now available to watch on Disney+ in the US and the UK.
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