Is Niagara Falls shrinking?

Short answer: No, Niagara Falls is not shrinking in any way you'd notice as a tourist. The water flow is heavily regulated by international agreements, so what you see is basically a controlled show. The falls naturally erode about 1 foot per year, but that's been slowed way down by diversions for hydro power. The American Falls and Bridal Veil Falls lose more rock because they're shallower, but the Horseshoe Falls (the big one) is actually getting a bit taller over time as the plunge pool deepens. You're not going to wake up tomorrow to a dry gorge.

If you're asking because you saw some clickbait headline about "Niagara Falls Drying Up," that was a one-time freak event in 1848 when ice jammed the river upstream, or more recently in 1969 when the US Army Corps of Engineers temporarily shut off the American Falls to study erosion. That's not happening again. The only thing shrinking is your wallet if you buy overpriced stuff on Clifton Hill. Bottom line: the falls are fine, go enjoy them.