Particulary relevant bit that made the rest of the topic more accessible, to me:

The researchers noted that a backlash against apps being marketed to catch cheating spouses, for example, has seen developers switch to marketing apps as parental control tools instead.

Ms Eva-Maria Maier, first author of the study from St. Pölten UAS, said: “The key issue with the extensive functionality of these unofficial apps is consent. If a parent has an open, transparent relationship with their child, they shouldn’t need to hide them on their child’s phone or have access to so much private information.