No, no hard facts (there isn't a hard fact on this entire thread). I do, however, have a rather large circle of cruise addicted friends across the nation; and there is no doubt, in this cross section, there has been a major shift in participation.
We have a travel group that cruises together every February, 23 couples.
Historically all but 3 couples participate in the auto gratuity. For 2024, Odyssey sailing that number has shifted dramatically, currently only 7 are planning on utilizing the plan. This is a multifactorial issue, no doubt, but the latest increase was the straw that broke the camel's back for many.