I can tell you that it was around 2009.
Here's the deal. See, traditionally, wrestlers would start out at the bottom and work their way up. As they got more exposed, they'd ideally get more over. The reward for their overness was a chance at holding a title in their slotted division. Only the top guys in each division would get or have a chance at getting a title. Bret Hart won his first singles title at Summerslam 1991-8 years into his WWF run. Somewhere around the early 2000s, WWE lost their way and figured "hey, forget having the competition elevate the titles, let's have the titles elevate the competition." They figured that just by slapping a few title reigns on a guy, he'd get over that way instead of organically. Since the titles meant something, if put on a guy who wasn't over, by virtue of holding the title, he'd mean something, too. It was a synthetic, fast track method of success. And that's why it totally failed. It gave us jokers like Jack Swagger (who I'm fine with, but in 2010, it was a mistake), Alberto Del Rio and Sheamus. Guys who really had no business at the top were shoved in while the audience was TOLD they were stars. As a result, this greatly devalued pretty much every single title on the roster.
So since the WWE title meant shit (even less with there being 2 world titles during the brand split that was still going on in 2009), people started looking for a new metric to judge whether or not a guy was getting a serious push. Holding a top title was no longer a useful measuring tool, since even losers were getting title reigns. Well, it became apparent that there were still 2 metrics that could give you this information. One was ability to main event PPVs, especially big ones like Mania or Summerslam. And the second was whether or not you could beat a top guy clean. A clean win is one where the loser becomes a lot more exposed due to the loss being due to them simply not being as good. Their reputation is vulnerable. Top babyfaces are the biggest draws and money makers in a wrestling company. That's because the fans have been groomed to recognize them as worthy of attention and admiration. This is usually achieved by the fact that they don't lose often, and if they do, its because the babyface is so great the heel had to cheat to beat them. Top babyfaces rarely lose clean because it can potentially hurt their drawing power and money making potential. So as a result, if someone is given a win over a top money maker/drawer, its a clear indication that the WWE wouldn't book that unless they were really willing to follow through with that risk. If they think you're worth putting over a top money maker/drawer, then they see value in you, which can translate into better booking, better feuds, more choice angles and generally better treatment. They are essentially investing in you by giving you a huge rub, which they hope will eventually pay off by turning you into a money maker/draw. And since screwing with your investment is stupid, it tends to mean that they'll get their shit together when putting together your future and give you favorable treatment. Fucking with a prop belt is whatever. Fucking with your cash flow is serious business. As such, a clean won over a top babyface started to become what it has ended up as now-one of the only real, surefire ways to gauge whether or not a guy is slated to get preferential treatment, which if he has fans, is likely to thrill them. Everyone wants to see their guy get used to his full potential-it means better entertainment for them. On the other hand, if he has a lot of haters, it'll provide said haters with infinite fodder to bitch about online in regards to how someone else who they like ACTUALLY deserved it.