The buy-back percentage varies a little from year to year and event to event, but a good overall figure across the 3 events over the period when I have been calculating it (the past 11 events) is that about two-thirds of the entrants use the buy-in option. It's generally a bit higher for 9-Ball Bank than for One-Pocket and 9-Ball.
And, no, it has not dropped over time. In 2023, for the 3 events combined, it was just a tad under 70%. This year, I was unable to calculate the percentage for 9-Ball because the Completed Matches list for 9-Ball was not posted on line. But for both Banks and One-Pocket, the percentages were higher than in an other recent year; let's call it three-quarters instead of two-thirds for those two events this year.