In order to use Windscribe on an iOS device, you need to be running a minimum of iOS 10 or above. However, keep in mind that certain application functions are available to higher versions of iOS.
For instance, in order to connect to Windscribe via the Wireguard protocol, your iOS device must be running iOS 12 or above. If you are unsure about the specifics of your Apple device, have a look at this help article: https://support.apple.com/en-us/HT201685