This also happened to steam a few days ago. They even changed their game policy stating that their payment processors determine the content allowed on steam.
That's true, but at least Steam did not hide NSFW games from search/recommendations (if you had mature content enabled), which Itch.io did without warning. And I assume that NSFW games likely represent a far higher % of revenue/user activity compared to normal games for Itch than for Steam.
Mind you, the exact same situation steam faces right now, itch.io had been facing ~3 months ago. If the current state of itch.io is anything to go by, things are still likely to get worse for steam.