[floo]

According to apple [1] ISRG Root X1 is available all the way down to iOS 10. So that would be every iPhone since the 5.

As far as marketshare goes iOS 13 and 12 make up 94% of devices [2]. So I am guessing its an insignificant amount of actual users.

[1] https://support.apple.com/en-us/HT204132

[2] https://developer.apple.com/support/app-store/