Prop bets have been around for a long time. They tended to happen in sports bars more than anything. Guys starting with “5 bucks says”. Prop bets at the bookie generally had to involve the whole game.
But it’s not the specific kind of bet that matters. It’s the rig. And rigs of various types have happened for ages. When all the betting is illegal they usually get found because somebody involved gets busted for something else. Like the betting scandal Henry Hill’s (later slightly fictionalized in Goodfellas) was involved in. That didn’t fall until years later.
But with legal betting, even in the old days when that was just at sports books in Vegas. The house has a stake in things not being rigged. They pay a lot of people to do a lot of math to set odds and lines such that the house always wins. They only lose money when somebody puts their thumb on the scale. So when their math spots that, they call the feds. There’s an ESPN 30 for 30 on a basketball scandal from the early 90s (too much work for me to look it up). And the investigation started with a sports book in Vegas saying “this looks funny”. Legal gambling is better at catching cheats. Which is why we’re getting all these stories so soon after they happen rather than the usual decade or more of the old days.
Its better at discovering cheating because they can use algorithms to monitor where the action is coming in and see suspicious, unexplainable last minute surges in betting on a specific bet. In just about every instance, though, it takes a snitch to nail the athlete. The athletes are to dumb to know the people they are dealing with are very prone to getting arrested and will rat them out in a second to work down their case.