I'll take the concept from @
ThrashtheTrash's post. The Jean era is a downwrad spiral.
Seasons 13-16 were pretty good. I dare say
The Simpsons recovered part of its enchantment after two bad seasons. Despite some bad troops, a handful of horrendous episodes, and even though this era was a step down in comedy aspects, I can see more consistency, some neat ideas, and way better and more stable characterizations. I'll take another concept from above and say these seasons were "traditional". You can make a solid argument that any of these seasons was the best since season 9.
Then season 17 was the first downgrade. The plots become repetitive and predictable, guest stars got more prominence, and there was more filling episodes from there than in the first four years of the show. Season 17 was largely discussed in this forum and I think it's more or less in the same tier as the next four season (maybe 20 stands out as the best, but it's not comparable with 13-16). Yet there were more decent episodes than awful ones, and a a few outstanding episodes per season.
But season 22 was another break point. The Al Jean's NABF season was awful. Call it a coincide or a consequence, but this was the first season of Matt Selman as a co-runner. The most remarkable episode was
The Great Simpsina, a very low episode to be the best of a season, and many episodes fail in every level, like
Love Is a Many Strangled Thing. From that point until season 30, there were more failures than successes from Jean, with two or three atrocious episodes per season and maybe one every two year to truly stand out. These years are plenty of mean-spiritism, flanderizarion, inconsistent characterizations, cartoonish moments, and shallow and confusing plots...
And yet it was better than what was yet to come. Jean's season 31 and onwards are unwatchable except for
The Way of the Dog and some barely decent episodes. These episodes don't even have coherence. Plots are a mess, there's lack of ambition, lack of cohesion, lack of ideas. There are lot of low-quality reharsh, ineffective callbacks to classic episodes, lots of cringeworthy moments, predictable jokes, and forced dialogues. A current Al Jean's episode is a compilation of unfunny jokes sticked out together in incoherent ways. It's terrible to the point I haven't watched his episodes from season 34 – and I have watched his episodes from season 35 without knowing he was the showrunner and needed one minute to be sure it was him because it just looks unprofessional and unambitious. To answer the poll, this is the point I'd say it turned awful, and maybe season 17 was when it turned predictable.