You probably won't find direct proof any more than you will find direct proof of any random VPN selling your data, it's just a given that commercial entities are liable to sell financially valuable data, and a list of all traffic, every website you visit and every service you use, tied to a specific identity is certainly financially valuable. Being in the EU doesn't change this; in fact the EU explicitly required that ISPs retain your identifying data with the Data Retention Directive, and though this was struck down after 8 years in court, many individual national governments immediately moved to impose similar requirements. I don't know if Germany was one of them but unless Germany has a specific privacy directive that goes beyond EU law I would see zero reason to place any trust in an ISP. In fact even if there was a law that's still not a reason to trust an ISP, because privacy laws are violated constantly; the most trustworthy source by far is a party acting opposite to the government, who has been investigated by the government and proven not to log the data that the government wants.

Even if it farms and only stores your data (which it does) without selling isn't good. YMMV between EU countries but I think even torrenting in Germany is way less safe than eg. in Poland where nobody bats an eye.