Since Steam does not meet the requirements to limit access to material to an audience above the age of 18, Valve has decided to remove access to such software in the region. It is compliance with law by exclusion. Consider it the same way you cannot access the online stores of several US supermarkets and other companies; they are complying with European GDPR by excluding your access. That way, they do not need to ensure your personal data isn't processed. Likewise, by excluding you from purchasing this game on Steam, Valve does not need to verify your age - and thus complies with German law.
I found out myself that the reason (maybe) why this happened is some stupid new law that requires developers that sell their games on Steam to have a age rating on their own ( the Steam age rating isn't enough for germany). I startet reasearching after i noticed that you can't buy FNAF 1-5 anymore on Steam in Germany.
That is something different, but related.
Basically: To sell porn games in Germany you have to verify the age of the customer. Steam doesn't do this, at least not in a serious manner. Hence Steam blocking all porn games in Germany, so they don't have to change their age verification system.
The issue with things like FNAF 1-5 is that a lot of older games do not have any age rating set at all.
Just the developer setting their age rating to something valid will fix that issue.