if I update to iCloud drive would I still be able to backup an old iPhone 3GS running iOS5?

It seems that with iOS9 switching to iCloud drive would be mandatory for some application that save data in iCloud (in my case Goodnotes). I should switch therefore on iCloud drive but I still need to do backup on my iPhone 3GS and I got a message from iOS that files would be not accessible by the 3GS if I update.