No-one has replied because you haven't provided enough information. First - are you running an aftermarket (AM) exhaust system? Second - when you say 'backfire' are you referring to ignition backfire through the FI system (the true definition of backfire), or exhaust popping? Exhaust popping on deceleration is natural and always happens but it is muffled by the errrmmm....original MUFFLER - hence the name. If you have an AM system then the popping is more pronounced. You can eliminate some exhaust popping of an AM by slightly enriching the fuel mixture, but you will never get rid of it entirely....and if you are running an AM system - make sure the Lambda sensor is installed because it will adjust the lean air/fuel ratio and also help with eliminating the popping..
I feel that the question "Anyone install one yet? " is pretty straight forward wrt 2017+ Ninja 650. My bike is 100% stock (not even tail tidy), and it's the same intake backfire that some Z650 owners have (not exhaust pops). I was hoping someone else had installed a PCV on the latest gen before I pull the trigger. They claim it's compatible with the latest gen, but it would be nice to have an end user confirmation. Again, all I'm asking if anyone has installed a PCV on the latest gen N650. I suspect nobody has replied because most people don't bother to install PCV on a commuter N650 -- riders chasing power get a different machine (especially since the latest gen has less peak whp than the previous gen.)