If I've been involved in the public El Capitan betas for months, there is no reason Sling couldn't have as well. The fact that Sling doesn't have a release posted for this today when the OS is released is yet another nail in Sling's customer support coffin.
Lord I wish I had just bought one of the newer Tivos that support streaming. Let's see how many days/weeks it takes for this to get addressed...
They did the same thing when Yosemite was released. There is no reason they could not have downloaded the open beta and tested their systems with it. They always seem to be one step behind instead of one step a head of things. If just switching the browser agent tricks their system to make it work, then they are totally doing things the wrong way. Stop relying on a browser agent string!!