Big Time Gaming is usually credited with popularising the mechanic through the Feature Drop in White Rabbit in 2017, though a few earlier games had something similar hidden away. Back then the idea was controversial and plenty of forum regulars called it a gimmick that would not last. The timeline of how the feature spread across studios and regulators is covered on https://thebonusbuyslots.com (https://thebonusbuyslots.com), and it is interesting to see how quickly the pricing settled around the 100x standard. Whatever you think of it, the mechanic is not going anywhere.