It is not new at all. As far as I am aware, it was added to CS6 back in 2012 after it had become crystal clear that NOT adding it is entirely confusing, since png is a standard web format, and a method had to be introduced to differentiate between those and the layered png file format Fireworks uses to save its native files.
Even before CS6 Fireworks users were already applying this double extension when saving their FW files - it sort-of became the standard.
Some voiced their concerns about this compromise, since the .fw.png double extension goes against the accepted rules, and those users wished for a .fwd or a .fire native extension.
At this point the discussion is over and done with, of course: there is no Fireworks CC, nor will there ever be one, since Fireworks development was halted by Adobe in May 2013. No more support or bug fixes are forthcoming either. It ended with CS6.
I'm moving from CS3 to CC so it's new to me. I will still use FW even though there are supposedly alternatives. I just love how intuitive it is.
Yes, it is a great application. A real shame it was discontinued - I would have loved to see a modernization of its web features and prototyping. Sad Adobe decided to pull the plug on FW. :-(