I’ve googled it, and some people report that Steam Keys don’t work for banned games in Germany.
Weird, for me it worked many times before and I live in Germany xD Well the US person bought me the key for KoboldKare and then just gave me the code and it worked(since you can't search the game on Steam Germany). Why would the code be any different though, if I bought it myself on itch.io and enter it on Steam? Probably also works then if I got it myself instead. I also got gifted F.E.A.R keys, despite the first game and the violent second one being banned in Germany and don't show up on Steam store. Websites like instant gaming also offer these F.E.A.R 1 and 2(violent edition) for German folks. Apparently the reviews say that it worked for them? The Steam keys they got from there.