"Extraneous subplots" refers to additional storylines or plots in a book, movie, or TV show that are not necessary or relevant to the main story. They can make the overall narrative confusing or distract from the main focus. Full definition
The narrative's blandness is only highlighted by a fresher but extraneous subplot in which Ella sleeps with their noxious English teacher (Kieran Culkin) without any punitive or emotionally scarring aftermath. (sensesofcinema.com)
It's a decision that forced the writing team to add extraneous subplots, a few new characters and even a romantic subplot to the story. (johnhanlonreviews.com)
Credited to Steve Oderkerk (Nothing to Lose), Barry W. Blaustein and David Sheffield (The Nutty Professor), and Paul and Chris Weitz (American Pie), The Klumps spends too much time on extraneous subplots and stupid jokes. (haro-online.com)