That's correct; adult games are banned in Germany on Steam. You'll be able to play the Itch.io version, which will be updated alongside the Steam version.
Adult games aren't banned on steam, because they are legal in Germany. Valve just wants to avoid implementing an age verification system, so they stopped selling the games themself by blocking the store page. Everything else is still in place. You just have to buy the keys from a different store. Since Valve has an office in Germany they can't claim that they just have to follow the laws of the country where their EU HQ is located.
No, Valve is blocking VPN Services for quite a while now. You not only need a VPN but also a payment method located inside the country you are using. More importantly: Using a VPN to hide your country violates steams terms of service and can result in the deletion of your steam account. Buying keys from outside steam and activating them is perfectly fine.