Yes, it is. That you don't necessarily have the ability to make that connection doesn't matter, although if it turns out you have it of course makes matters worse. (This also isn't new under GDPR, current european law interpretation already supports this. See http://curia.europa.eu/juris/document/document.jsf?text=&doc... for the court decision firmly establishing this: Since the visitors provider has the data, and will share this data in some cases, it's possible to establish the link and the dat thus has to be protected accordingly)