I dug into this because I love when movies trim a book but still keep the soul of it, and 'It' is a classic example. The films (both 2017 and 2019) cut a ton of material from Stephen King's colossal novel—most notably the huge, weird, and often metaphysical chunks that make the book feel like a full history of Derry.
The biggest omissions are the deep dives into Derry's past and the Ritual of Chüd as King wrote it. In the novel Mike spends pages and pages cataloguing violent events and disappearances in Derry across decades and centuries; the movie compresses that into brief montages and exposition, losing a lot of the town-as-character feel. The novel’s metaphysical battle (a long, hallucinatory mind-fight with cosmic layers and weird symbolic encounters, plus the Turtle/Maturin mythology) is basically replaced by a more direct, visual showdown in the films.
Other cuts: lots of adult Losers backstories and scenes that explain who they became (their careers, marriages, and inner lives) are shortened or removed. There are subplots about Henry Bowers’ family history and his descent into a more grotesque arc, Patrick Hockstetter’s disturbed behavior is given less room, and there are many small, creepy episodes (like extended PTSD/trauma scenes, townspeople's conspiracies, and weird side encounters with Derry’s oddities) that simply didn’t make the cut. The films often opted for sharper pacing and clearer horror set-pieces rather than the book’s sprawling tapestry of small-town rot and supernatural lore, so if you loved the novel’s depth you’ll notice a lot missing—but if you want a tight horror movie, the adaptation did its job.