How common is it for iPhone users to NOT be running iOS 10+?

I'm building an app in which I want to use a module that works only for iPhones with iOS 10 and above.

I know I can never assume that everyone is using a given version, but I just wanted some insight into how common it might be that someone would be using a version less than 10?

I've always kept my own phone up to date, but I don't know the repercussions and likelihood of someone not updating it.

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.}

According to Apple as of Jan 2020:

• 77% of all devices introduced in the last four years use iOS 13
• 17% of all devices introduced in the last four years use iOS 12
• 70% of all devices use iOS 13
• 23% of all devices use iOS 12

This is the only breakdown I could find from Apple. Other breakdowns might exist from 3rd party analytic sites.

It looks like there's a very small number of users on iOS 10, based upon n these stats.