It's generally recognized that The Simpsons drifted from sharp comedy to cosy light entertainment as the years went by, and that a threshold was passed somewhere between seasons eight and eleven. Using data culled from IMDB and a contiguous cluster analysis, Nathan Cunn pinpoints the exact end of The Simpsons' golden age to the half-hour: episode 11 of season 10. This particular way of seeing things condemns no particular episode's sins, merely putting a statistical dividing point between Wild Barts Can't Be Broken and Sunday Cruddy Sunday. Compare to The Principal and the Pauper, the season 9 episode traditionally identified as the shark-jumper, which in this chart is a controversial blip on the road to all the disengaged meta to come.
It's remarkable that the show managed to go for over nine seasons, and over 200 episodes, with an average rating of 8.2. The latter seasons, in contrast, have an average rating of 6.9, with only three episodes in the latter 400+ episodes achieving a rating higher than the average golden age episode—those episodes being Trilogy of Error, Holidays of Futured Passed, and Barthood. Given that the ratings approximately follow a Gaussian distribution, we expect (and, indeed, observe) that roughly half of the golden age episodes exceeded this mean value.
Although The Simpsons isn't quite the show it once was, the decline in the show's latter seasons is more testament to the impossibly high standards set by the earlier seasons than it is an indictment of what the show became.
Nonetheless, the author also posits that further declines in standards may be masked after a certain point by survivorship bias: votes coming only from fans whose perception of quality won't change so long as the quantity remains. On the other hand, internet ratings are not the be-all and end-all of America's collective critical faculties, either.