https://arstechnica.com/features/2018/06/why-the-supreme-courts-software-patent-ban-didnt-last/

Forty years ago this week, in the case of Parker v. Flook, the US Supreme Court came close to banning software patents. "The court said, 'Well, software is just math; you can't patent math,'" said Stanford legal scholar Mark Lemley. As a result, "It was close to impossible in the 1970s to get software patents."

If the courts had faithfully applied the principles behind the Flook ruling over the last 40 years, there would be far fewer software patents on the books today. But that's not how things turned out. By 2000, other US courts had dismantled meaningful limits on patenting software—a situation exemplified by Amazon's infamous 1999 patent on the concept of shopping with one click. Software patents proliferated, and patent trolls became a serious problem.