He didn't.
He showed up in Diatribe of a Mad Housewife telling Marge he was very sick.
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There is no evidence any retired characters will be returning The Simpsons at least in any recurring capacity that they require new voice actors.Edna Krabappel died off screen during Season 26.There's no reason to believe Dr. Marvin Monroe will be returning if doing his voice caused suffering for Harry Shearer.There's also no reason to believe Capital City Goofball will return. He was a one shot character who served no real purpose to the show so there's simply no logical reason they'd bring him back.Finally Troy McClure, after Phil Hartman's death, out of respect they retired his character. They aren't going to revive him.
According to Wikipedia - it's Dr. William McDougal,
I can't find such a character. There's Sideshow Bob, but ever though he has a lot of hair, it's not exactly long.
Yes Dr Laurie Kinkaid a biology professor at University of Louisiana Monroe though she is probably retired
i dont no