I can tell for sure the current version of StreetComplete for Android does allow specifying that only one side of the road has a side walk. I solved a few of these quests just last week. No idea what's the state on iOS though.

On android there's a "differs along the way" option which let's you split the way.

There is a limit to the type of quest you can fit into the mobile app. It also should have a fall back "leave a note" option.

In this case, the intended flow is that you tap "Uh..." -> "Differs along the way..."

You will then be led to another UI in which you can split the road into several sections.

First-time StreetComplete user here. I just answered a few of these "street has a sidewalk" questions myself. Upon first clicking on one of them, I was able to answer this question for each side of the street individually. The visual selection that you mentioned only applies to one side.