I don’t think there’s anything wrong with storing anonymous data: a log of bookmarked urls for instance. So long as there’s no way to trace back who they belong to if that person wants to be forgotten.
But can he be identified ? I believe if its too difficult to identify using even "religious beliefs, sexual orientation, political views etc" then its not personal data. Though I cant seem to find link to the page where I read it.
On similar note, even IP _alone_ appears to be non personal data if since it cannot identify a person [1].
From Case 582/14 – Patrick Breyer v Germany [2]
On appeal, the Regional Court of Berlin (the "Kammergericht") ruled that IP addresses in the hands of website operators could qualify as personal data if the relevant individual provides additional details to the website operator (e.g., name, email address, etc.) in the course of using the website.
Lol even German Govt cant seem to figure out its own laws. They should have gone to one of those consultants.
[1] https://www.gdpreu.org/the-regulation/key-concepts/personal-...
[2] https://www.whitecase.com/publications/alert/court-confirms-...