How common is it for iPhone users to NOT be running iOS 10+?
how common it might be that someone would be using a version less than 10?


Based on the publicly shared data by Apple (linked below), a very few percentage of currently active iPhone devices are a version of iOS that's less than 10.

So if your question is along the lines of, how many potential users do I lose if I drop support for iOS 9 and earlier, the answer would be a very few.

This should help you get a perspective:


App Store - Support - Apple Developer



  


The webpage is updated by Apple on a regular basis. The statistics are drawn from currently active devices (ones which have an active Internet connection and visits the App Store app).

The snapshot above is captured at the time of writing the answer.