‘When did The Simpsons jump the shark?’ is a loaded question with multiple answers (FWIW, I don’t think it was the episode where Principal Skinner revealed his true identity). But if you wanted to use empirical data to back up your claim, now there’s a site to help.
SeriesHeat displays heatmaps of every episode from popular TV series based on IMDb ratings. Here’s one for The Simpsons:
As you can see, Seasons 3-8 were the golden years and things started turning from Seasons 10-11. There’s not been a ‘green’ score since Season 12 Episode 18, which aired in December 2000.