About 5 months ago USK, the German regulatory agency for age ratings in video games, warned steam that every game should have an age label or be hidden/banned on the German store.
When the deadline arrived lots of indie-games disappeared off of peoples wishlists and while most reappeared a few days to weeks later, some have stayed gone since then and I can imagine that those text based games were hit by that too and simply don't have the resources to get them rated(or maybe their devs don't even know).
And if by 18+ you meant porn then yeah, Germany has always banned those from German Steam stores because steam has no age verification needed to get access to them.
I hope this helped in some way.