It’s not a scrollbar, it’s what Apple call the “home indicator”, and when it was introduced they strongly recommended app authors don’t hide it except for things like videos, as they fear users will get confused. It’s been a few years since the introduction though, so I can look at changing this for the study screen. Note that there’s no “off” setting, there’s just a setting that tells iOS we’d like it to be hidden when iOS decides it would be appropriate to do so.