1. Brooklyn Nine-Nine started off as a workplace cop procedural sitcom with an emphasis on the characters personalities, which allowed for varied plots with interesting guest stars playing people involved in the crimes (like the amazing interrogation-based "The Box" with Sterling K. Brown as the suspect) or even just Police bureaucracy, though over time it seemed to become more and more about their personal lives, with the Jake and Amy relationship as the main focus. It never fully abandoned the cop stuff, with the final season even getting into BLM related topics, but the shift definitely seemed to favour more standard sitcom material based around personal lives. The show could have even still worked the premise into that with the characters who were married to civilians. Terry was introduced with a wife and kids for instance, but didn't seem to get as much focus as you would expect, and Holt's relationship with his husband Kevin also seemed underexplored, especially with the whole political aspect of how a gay black police Captain would have been treated.  
2. GLOW was a 1980s set dramedy inspired by a true story about an all-female pro wrestling show ran by a cynical director with a struggling actress and her more successful estranged best friend as the two biggest stars. Season 1 dealt with the training and production of their very first show, with season 2 being about how they built on that as an ongoing series and how they developed the characters and storylines before being cancelled. Season 3 however had the show being adapted as a live Las Vegas casino show, which was made into the same thing done every night, which resulting in the wrestling aspect being put into the background and the drama aspect of the characters' personal lives getting more focus than ever.   
3. Riverdale started as a tongue in cheek take on the Archie franchise, a high school drama about the characters from those comics, with some dark thriller twists thrown in. Things got heightened, with topics like murder, drugs and even sexual assault being covered, but over time got more and more ridiculous and less realistic. Evil cults, decades long secrets, a time jump that made Archie a war veteran with PTSD and Betty an FBI Agent tracking serial killers (which she specialised in because her own dad and brother turned out to be those), until eventually there's a plot involving the Devil and a comet and the final season takes place in an altered reality where the characters are all back in high school, but in the 1950s, like the original Archie comics.

Show abandons or downplays its main premise over time