Been like that for well beyond that, 20+ years (probably since the first porn site went online). In the late 90s, early in the career, I was brought into an adult website project. You were limited to working with a few payment processors that were willing to work with the industry, and the integration was pretty rudimentary, even for the time.

Note that it's not just a morality issue: porn sites have a tremendous number of chargebacks. As a result, payment processors ban categories where the risk is higher.