The latest Overwatch patch brought out an odd Scatter Arrow cool-down decrease from ten seconds to eight seconds. This information was not put anywhere in the most recent patch notes and was uncovered by user Emc73.
A while back, Jeff Kaplan announced that the team was working on changing Hanzo's Scatter Arrow completely into a new ability. The name is still currently pending but, Rapid Shot was the most recent iteration of the ability that would take over Scatter Arrow. In addition to these intended major change, Sonic Arrow is being looked at as well.
Since then, there has not been much information about the status for Hanzo's kit rework. The hidden buff to Scatter Arrow is a strange one since it was intended to be removed all together. It's possible that eight seconds cool down was intended for Rapid Shot and players could see the Hanzo changes in the near future.