Kingston HyperX Savage DDR4 RAM is performance RAM between HyperX Fury (value RAM) and HyperX Predator (enthusiast RAM). It was created for Intel 100 series and X99 chipset.
Back in the end of 2016 when i bought my Skylake build (full specs with pics in my sig), i also went with HyperX Savage RAM due to it's performance over value RAM while still being cheaper than enthusiast RAM. I have
HX430C15SB2K2/8 in use. Once i decided to have more RAM in the middle of 2017, it was pretty hard time for me to find another set of HX430C15SB2K2/8 RAM. Though, i did manage to find it eventually.
As far as why HyperX Savage isn't available anymore, well, my guess is that most people went with value RAM (HyperX Fury) and Kingston just didn't produce HyperX Savage RAM that many. Low sales = small production number. Then again, HyperX Savage RAM is one of the best out there and i can see why no-one is selling it. Even i wouldn't sell my HyperX Savage RAM, despite having 4x sticks of that RAM in my system.
While you could buy 2x 4GB HyperX Fury RAM to get total to 16GB, i'd advise against that. Mixing RAM is bad idea since it's 50:50 chance of old RAM and new RAM working together. It's best when you buy any 2x 8GB 2400 Mhz set and keep your 2x 4GB set as backup (or sell it). Since your MoBo supports RAM speeds of 2133/2400 Mhz, you can get small RAM speed upgrade with 2x 8GB 2400 Mhz set as well.