‘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.
Since 2015, the site has remained mostly ad-free. I post affiliate links from time-to-time but I try to post alternative links where appropriate. I also write most of these blogs myself. If you read this and enjoyed the content you've so far, why not consider pledging to my Patreon.