I'm on Android 7.1.2 on my Pixel (on Project FI).
As far as I know, there is no way to restrict individual apps from using cellular data at the system level. That said, most GOOD apps have the option within the app to restrict data usage to Wifi, and even wifi only when plugged into power. Google Photos, maps, netflix and many others have similar options. It would be better to allow this control centrally like iOS does.
One area where Android is WAY better than iOS is the ability to define a data cap. I have a cellular data cap of 5gb set right on my device, to ensure I never go over 5gb of usage per month.