For instance, the Internet's digital distractions and supernormal capacities for cognitive offloading seem to create a non‐ideal environment for the refinement of higher cognitive functions in critical periods of children and adolescents’ brain development. Indeed, the first longitudinal studies on this topic have found that adverse attentional effects of digital multi‐tasking are particularly pronounced in early adolescence (even compared to older teens), and that higher frequency of Internet use over 3 years in children is linked with decreased verbal intelligence at follow‐up, along with impeded maturation of both grey and white matter regions.
This paper doesn't mention IQ even once. It just discusses general effects it can have e.g. potential problems with multi-tasking for some, possible beneficial effects via increased mental stimulation for the elderly etc. This is quite different than what you initially proposed, not to mention inconclusive.
What do you think cognitive decline means? IQ tests are also called cognitive assessments and the accurate ones (not the ones you take online) involve a lot of different aspects of assessing how your brain is working. Many psychologists don't like the term IQ because it's too basic of a term...which is why many papers studying the subject do not include it.
