Haven't bothered with much television in a long time. But, the most obvious answer is GoT. Season 5 onwards it was a gradual decay with time alotted to useless plots and more and more convenient closures to existing plotlines.
TWD is another show I'd put up here. Dropped that shit in Season 5. The literal world building was interesting but the main appeal is the zombies. When it becomes recycled drama with the zombies being treated as a backdrop, so uninteresting and not a threat that the writers made them appear in boring scenes just for quick action, I dipped.
Scrubs. Watched this religiously growing up, loved the show so much. But I do remember.... It dipping near the end. My mind is hazy on the last few seasons, but I know the final season featured almost a whole new cast. Which was shitty.
Speaking of new casts, add in That 70s' show and the Office (U.S. addition).