That is 100% not true as HoneySelect2Libido DX is on steam as well as a bunch of other 18+ games. Unless you are of course talking about games made with Honey Select 2 client, that is a different story.
18+ games isn't the issue, Honey Select is
It's a copyright issue. HoneySelect2Libido DX is sold on Steam and any games made with the Honey Select 2 client are banned for two reason.
1. The copyright holder of HoneySelect2Libido DX is litigious and prone to sue for copyright violation.
2. Games made with Honey Select 2 are considered mods of HoneySelect2Libido DX and Steam doesn't allow mods to be sold.