You complain about different things.
a) Your game is considered nsfw on Itch. And in my opinion, it is rightly so. If your game's certification in any given country would be adults only, all ages, 12, 15, 16, 17, 18 or 20, does not matter at all. The problem is, Itch has no age certification for games. They only have nsfw. Which usually includes pixel nudity in low res.
Personally from the little I saw, I would put it somewhere between 12 and 15. But adjusted for US based Itch, I would put it at 18, since there is only the choice of 13 and 18.
You try to publish on Steam. They will give your game an inofficial rating. Might be interesting, what rating they will give it for different regions.
b) Paid nsfw games are currently not indexed in general. We all hope this will be changed. Does this concern you? Not really. You have an unpaid demo version you can use to be visible. But apparantly you do not want to do this. It is delisted, either by yourself, or because you removed all files.
Any accidental traffic you can catch with the demo version and guide people to your paid version. Just do not try to mark your demo as sfw, when it links to a nsfw game. Even if you disagree with the nsfw attribute.
And any direct traffic you generate outside from Itch does not care if the game is indexed or not.
c) You see some games that are nsfw and are paid and are indexed. There is several hundred of those and they might be staff aproved or are indexed for some other reason. Itch has no issues with indexing adult/nsfw content as such and there is no rule that paid nsfw games cannot be indexed ever. Read here https://itch.io/t/5149036/reindexing-adult-nsfw-content There is actually written the intent to re-index paid nsfw content.
They just do not do it currently for most games. Maybe for some games it is a combination of the used payment methods, or some other factors. In the thread the problem seems to be Stripe. Itch usually does not discuss certain things in public. Or at all.