Odds are, it's a change the payments cards company (Mastercard, etc...) or payment processors is forcing on them. Probably following the issues they had with Paypal and the forced removal of incest related adult games.
Must be really recent since they never had a problem with adult game in Early Access before. There still are some adult games in Early access (Carnal Instinct, Sapphire Safari, etc...).
Latest released adult only game in early access was released on the 25th of August 2025.
We will have to see if other games are blocked or if it's only yours.
One option would be to have a "safe" version for Steam with a patch existing outside. That's what some Japaneses adult/erotic games are doing when publishing on Steam
Worse, come to worse, just release the game not in early access and add somewhere in the title or the description that it is a work in progresss.
A bit stupid, but the situation as a whole is stupid and if it work, it work.
Fuck Collective Shouts and companies like MasterCard.