I tried windscribe. That one didn't work / didn't fool Steam. Also, if possible i'd like to avoid using VPNs, since accessability of purchases in the future may change with differnt geoblocking methods.

Also, i'ts perfectly legal to sell adult only games in Germany. You just need a working(!) age verification. Which steam currently does not have. And no, "how old are you" drop-down menus do not count. ;)
They got away with it up until a few months ago. Then our authorities sent a strongly worded letter and Steam just geoblocked everything and made a noncommitant press release about looking into bringing them back in the future.

Well how would you implement it then? It's a digital store so nobody is really able to for example verify authenticity of your ID.

Same methods we use for online stores like Amazon. Theres PostIdent and our ID cards are chipped and support electronic authentification. Unfortunately, using these options means additional effort an Valves end.

Well here's the question: how could Valve prevent usage of network proxies for just Germany (to prevent circumventing the ID checkup) without explicitly yelling "no vpn" at the entire worldwide userbase? (if they would do that then that would majorly suck for several reasons as there ARE legitimate usecases for using vpn with Steam for things different than purchasing)

Also, nagging Steam support to implement it could maybe send a message to them eventually.

And I'm somewhat afraid that if Valve would implement it other countries could go all "oh, me too" and Valve would have to implement per-country separate systems... which would suck for a lot of people...

Also, does the ID system in Germany mean that technically gov knows what age restricted games you play?
If so then it's kind of privacy unfriendly :S

Hey, I'm not the guy to solve Valves problem for them. Or well, if I was, I'd negotioate a salary first... ;)

And no, our government does not have access to our purchases - unless the stores share this information. If they were so inclined, they wouldn't need the link to our ID card for a hard ID on the real person behind the account. Unless you've been paying anonymously with bitcoin, your payment info leads back to you.

Paying and liquidating crypto is profoundly hard to do without living any personal trace upon chargeup/cashup (if you know what I mean) at some point.

But dang, laws in Deutschland are getting more and more weird other time :P

edit: fixed typo

It is about that time of the century... It'll get worse before it gets better.