Imagine you are a baker in the end of 1930's Germany. You deliver bread every day to a synagogue. Imagine cell phones and apps existed. The Nazi government could now with little effort see you went to a synagogue everyday for the last couple of years so they decide to send you to a camp, although you are not a jew. Meta data is not dangerous you think?
There's no need for theoreticals - we know very well that Nazis used census data which recorded a person's religion to find and kill Jews(and others). At the time I imagine giving this data to the state felt like not a big deal, but how could they know it would lead to their deaths?
