In hindsight, it seems obvious to me that app developers would abuse "Background App Refresh" functionality of iOS to call home and send data.
So I went to Settings on my phone and found a giant list of apps with the Background App Refresh function turned on.
I turned almost all of them off. Especially those from companies whose business model relies on collecting my personal data: anything made by Google, Facebook (WhatsApp, Instagram), Amazon, Uber, Yelp, etc. Obviously, all those crappy adware/games my son downloads.
I only left the setting on for apps made by companies I trust because their business model is not about collecting data (Apple and to a lesser degree Microsoft) and only for apps I use often and I can see the value for the contents to be up to date by the time I open the app (Notes which may change on a separate device, Outlook, etc.).
The best part, this should improve my battery life – to what degree it's unclear and reduce my data usage.
I bet that Apple will, in the near future, default this setting to OFF and make apps ask you if you really want to grant them the ability to perform Background App Refresh.