How to Install Safari 9 on Yosemite?
I found that Safari 9.0 for Yosemite is indeed available, but first you have to upgrade to the most recent Yosemite version 10.10.5

In my case, I had 10.10.4 and the update for Safari was not showing up in the App Store. I wanted to update all at once but first you have to do the system update.