With shortening attention spans and the way we consume content nowadays, is there any shift in the way modern fiction books are written? Do contemporary writers write differently than those living in the non-digital era?
Are writers changing to accommodate short attention spans?
Yes, in some ways. The flash and micro flash formats seem a product of the digital age and are written to convey a complete story in 100-1000 words.
But, General Audience novels and short stories aren't shorter. The reading level of the language might be a little lower, but that is because of an attempt to reach a broader audience. And style has certainly continued to evolve and change as what is considered more interesting has changed.
For instance, Sci-fi from the 1930-1950s was much lighter on characterization. Today, character-driven stories are the fashion for most pro-rate publications. And it will change in the future to something else.
I think that there exists this impression because of the writing they see on most advertising-supported web pages. These are intended to be read quickly and are reaching a broad segment of the population -- like tweens and teenagers to retirees. Since the goal of the publishers -- or web site -- is to push as many advertisements in front of as many eyes as possible, they don't want people to linger over a story. They want people to read, share and repeat.