The overlays are intentionally a bit hidden, since they're more of a power-user feature and I appreciate that StreetComplete retains beginner-friendliness as a core tenet. But the overlays are indeed awesome for survey-type walks where you just want to make sure that all sidewalks are mapped correctly, or to see which shops are still missing.

Sort of. It was added in 2023. (https://github.com/streetcomplete/StreetComplete/pull/5373)