I happened to be a fullstack developer when the transition happened, so I saw it firsthand. I would say it predated V8 by a year or two. By the time V8 came out, I was already writing plenty of (simple) javascript for applications.
I would say it was more about plugged security holes and Ajax becoming more viable for real-world use. The “don’t ever use javascript” rule came from people disabling javascript because javascript was being used for malware. V8 was a part of that transition and growth, but at least in my memory not the shot that started it all.
There were developers (and books) pushing Rails+Ajax pretty hard in 2007, a full year before V8’s first release in September of 2008.
salient_one