It appears that in the future, Itch will allow creators to opt out of payment providers, meaning that it’s probably on a per game basis, not per platform. That Itch and Steam are not making a per game solution now, is most likely because their current software doesn’t allow it and they need time to rework it. Itch has promised various changes already, Steam has been mum afaik.

Source for Itch: “For NSFW pages, this will include a new step where creators must confirm that their content is allowable under the policies of the respective payment processors linked to their account.”. itch.io/updates/update-on-nsfw-content