While It touched briefly on some of the killings that occurred about every 27 years in Derry, Chapter Two hardly mentions them at all, spending most of its nearly three-hour runtime concerned only with the events of 1989 and 2016. But the book takes great pains to show us the extent of the evil that Pennywise is capable of by walking readers through Mike Hanlon's journal of past horrors.
Although the journal entries are extremely detailed, conveying grisly tales of fatal fires, mass shootings, and ax murders, they have little relevance to the rest of the plot, other than to establish what we already know: Pennywise is the kind of monster that even monsters are afraid of. It's easy to see why none of these interludes were included in the films as anything more than a passing nod, since they mostly just serve to disturb the reader, and not actually to advance the story.