Futurama: It's Always Sunny in Philadelphia

Season 13 was up against some pretty major production issues with regards to the availability to the cast. Most significantly was that Glenn Howerton was busy making AP Bio. That's why the season 12 finale sets him up to leave the show and the season 13 premiere plays on the idea that he's being replaced by Mindy Kaling.

Most of the season seems to have been designed to work around cast availability in general. The ladies reboot episode was very clearly written to allow for the male cast members to barely be needed. The two Superbowl episodes split the cast in half so that they could basically film an episode with just Charlie on his own and then film a separate episode without him.

I agree that it marks a significant downturn in the overall quality of the series but it still has some absolute gems. "Times Up for the Gang" might well be in my top 10 episodes. And I agree season 14 was also not amazing, but I think abysmal is a bit harsh. It's still solid TV and some episodes (The Gang Texts) were still great.

Season 15 actually marks the first season of the show after A.P. Bio was canceled and Glenn Howerton was pretty much back, full-time focused on Always Sunny. My guess is that you're picking up on his input and that he was perhaps less involved in the writing during seasons 13-14 (if not officially, he may have just been less committed to everything).

Season 15 was also produced during the pandemic which I'm sure did a lot to make them reassess what they were going to do with it and how they were going to approach it. The back-half being set in Ireland was, I believe, a production-driven decision above all else but it gave the season a real sense of direction and purpose.