Laurel and Hardy were already both deceased before "The Ghost of Bigfoot" from The New Scooby-Doo Movies aired. These were the only two celebrity guest stars in the franchise that had passed away prior to their episode being written and produced. (The Ghost of Abraham Lincoln was a historical figure instead of a celebrity, so I'm not including him here).
It's perhaps worth noting that a few guest stars also passed away shortly after their episodes aired. Both Larry and Moe of the Three Stooges both passed away in 1975, three years after their episode aired. Cass Elliot passed away in 1974, two years after her episode aired. Alex Trebek died after the production process on "Total Jeopardy!" had already finished, but before the episode aired.